diff --git a/m/m.go b/m/m.go
index ec15973..f456167 100644
--- a/m/m.go
+++ b/m/m.go
@@ -67,7 +67,11 @@ func ServiceContext() context.Context {
 
 // Start performs the work of populating configuration parameters and triggering
 // the start event. It will return once the Start event has completed running.
-func Start(ctx context.Context) {
+//
+// This function returns a Context because there are cases where the Context
+// will be modified during Start, such as if WithSubCommand was used. If the
+// Context was not modified then the passed in Context will be returned.
+func Start(ctx context.Context) context.Context {
 	src, _ := ctx.Value(cfgSrcKey(0)).(mcfg.Source)
 	if src == nil {
 		mlog.Fatal("ctx not sourced from m package", ctx)
@@ -76,12 +80,14 @@ func Start(ctx context.Context) {
 	// no logging should happen before populate, primarily because log-level
 	// hasn't been populated yet, but also because it makes help output on cli
 	// look weird.
-	if err := mcfg.Populate(ctx, src); err != nil {
+	ctx, err := mcfg.Populate(ctx, src)
+	if err != nil {
 		mlog.Fatal("error populating configuration", ctx, merr.Context(err))
 	} else if err := mrun.Start(ctx); err != nil {
 		mlog.Fatal("error triggering start event", ctx, merr.Context(err))
 	}
 	mlog.Info("start hooks completed", ctx)
+	return ctx
 }
 
 // StartWaitStop performs the work of populating configuration parameters,
@@ -89,7 +95,7 @@ func Start(ctx context.Context) {
 // stop event.  Run will block until the stop event is done. If any errors are
 // encountered a fatal is thrown.
 func StartWaitStop(ctx context.Context) {
-	Start(ctx)
+	ctx = Start(ctx)
 
 	{
 		ch := make(chan os.Signal, 1)
diff --git a/m/m_test.go b/m/m_test.go
index 0756cb4..e15d508 100644
--- a/m/m_test.go
+++ b/m/m_test.go
@@ -31,7 +31,7 @@ func TestServiceCtx(t *T) {
 		ctx = mctx.WithChild(ctx, ctxA)
 
 		params := mcfg.ParamValues{{Name: "log-level", Value: json.RawMessage(`"DEBUG"`)}}
-		if err := mcfg.Populate(ctx, params); err != nil {
+		if _, err := mcfg.Populate(ctx, params); err != nil {
 			t.Fatal(err)
 		} else if err := mrun.Start(ctx); err != nil {
 			t.Fatal(err)
diff --git a/mcfg/cli.go b/mcfg/cli.go
index ae104ea..c1cb9c2 100644
--- a/mcfg/cli.go
+++ b/mcfg/cli.go
@@ -17,6 +17,7 @@ type cliKey int
 
 const (
 	cliKeyTailPtr cliKey = iota
+	cliKeySubCmdM
 )
 
 // WithCLITail returns a Context which modifies the behavior of SourceCLI's
@@ -41,6 +42,45 @@ func populateCLITail(ctx context.Context, tail []string) bool {
 	return ok
 }
 
+type subCmd struct {
+	name, descr string
+	flag        *bool
+	callback    func(context.Context) context.Context
+}
+
+// WithCLISubCommand establishes a sub-command which can be activated on the
+// command-line. When a sub-command is given on the command-line, the bool
+// returned for that sub-command will be set to true.
+//
+// Additionally, the Context which was passed into Parse (i.e. the one passed
+// into Populate) will be passed into the given callback, and the returned one
+// used for subsequent parsing. This allows for setting sub-command specific
+// Params, sub-command specific runtime behavior (via mrun.WithStartHook),
+// support for sub-sub-commands, and more. The callback may be nil.
+//
+// If any sub-commands have been defined on a Context which is passed into
+// Parse, it is assumed that a sub-command is required on the command-line. The
+// exception is if a sub-command with a name of "" has been defined; if so, it
+// will be used as the intended sub-command if none is specified.
+//
+// Sub-commands must be specified before any other options on the command-line.
+func WithCLISubCommand(ctx context.Context, name, descr string, callback func(context.Context) context.Context) (context.Context, *bool) {
+	m, _ := ctx.Value(cliKeySubCmdM).(map[string]subCmd)
+	if m == nil {
+		m = map[string]subCmd{}
+		ctx = context.WithValue(ctx, cliKeySubCmdM, m)
+	}
+
+	flag := new(bool)
+	m[name] = subCmd{
+		name:     name,
+		descr:    descr,
+		flag:     flag,
+		callback: callback,
+	}
+	return ctx, flag
+}
+
 // SourceCLI is a Source which will parse configuration from the CLI.
 //
 // Possible CLI options are generated by joining a Param's Path and Name with
@@ -75,16 +115,56 @@ const (
 )
 
 // Parse implements the method for the Source interface
-func (cli *SourceCLI) Parse(ctx context.Context, params []Param) (context.Context, []ParamValue, error) {
+func (cli *SourceCLI) Parse(ctx context.Context) (context.Context, []ParamValue, error) {
 	args := cli.Args
 	if cli.Args == nil {
 		args = os.Args[1:]
 	}
+	return cli.parse(ctx, nil, args)
+}
 
-	pM, err := cli.cliParams(params)
+func (cli *SourceCLI) parse(
+	ctx context.Context,
+	subCmdPrefix, args []string,
+) (
+	context.Context,
+	[]ParamValue,
+	error,
+) {
+	pM, err := cli.cliParams(CollectParams(ctx))
 	if err != nil {
 		return nil, nil, err
 	}
+	subCmdM, _ := ctx.Value(cliKeySubCmdM).(map[string]subCmd)
+
+	printHelpAndExit := func() {
+		cli.printHelp(os.Stderr, subCmdPrefix, subCmdM, pM)
+		os.Stderr.Sync()
+		os.Exit(1)
+	}
+
+	// if sub-commands were defined on this Context then handle that first. One
+	// of them should have been given, in which case send the Context through
+	// the callback to obtain a new one (which presumably has further config
+	// options the previous didn't) and call parse again.
+	if len(subCmdM) > 0 {
+		subCmd, args, ok := cli.getSubCmd(subCmdM, args)
+		if !ok {
+			printHelpAndExit()
+		}
+		ctx = context.WithValue(ctx, cliKeySubCmdM, nil)
+		if subCmd.callback != nil {
+			ctx = subCmd.callback(ctx)
+		}
+		if subCmd.name != "" {
+			subCmdPrefix = append(subCmdPrefix, subCmd.name)
+		}
+		*subCmd.flag = true
+		return cli.parse(ctx, subCmdPrefix, args)
+	}
+
+	// if sub-commands were not set, then proceed with normal command-line arg
+	// processing.
 	pvs := make([]ParamValue, 0, len(args))
 	var (
 		key        string
@@ -98,9 +178,7 @@ func (cli *SourceCLI) Parse(ctx context.Context, params []Param) (context.Contex
 			pvStrVal = arg
 			pvStrValOk = true
 		} else if !cli.DisableHelpPage && arg == cliHelpArg {
-			cli.printHelp(os.Stdout, pM)
-			os.Stdout.Sync()
-			os.Exit(1)
+			printHelpAndExit()
 		} else {
 			for key, p = range pM {
 				if arg == key {
@@ -159,6 +237,23 @@ func (cli *SourceCLI) Parse(ctx context.Context, params []Param) (context.Contex
 	return ctx, pvs, nil
 }
 
+func (cli *SourceCLI) getSubCmd(subCmdM map[string]subCmd, args []string) (subCmd, []string, bool) {
+	// if a proper sub-command is given then great, return that
+	if len(args) > 0 {
+		if subCmd, ok := subCmdM[args[0]]; ok {
+			return subCmd, args[1:], true
+		}
+	}
+
+	// if the empty subCmd is set in the map it means an absent sub-command is
+	// allowed, check if that's the case
+	if subCmd, ok := subCmdM[""]; ok {
+		return subCmd, args, true
+	}
+
+	return subCmd{}, args, false
+}
+
 func (cli *SourceCLI) cliParams(params []Param) (map[string]Param, error) {
 	m := map[string]Param{}
 	for _, p := range params {
@@ -168,7 +263,12 @@ func (cli *SourceCLI) cliParams(params []Param) (map[string]Param, error) {
 	return m, nil
 }
 
-func (cli *SourceCLI) printHelp(w io.Writer, pM map[string]Param) {
+func (cli *SourceCLI) printHelp(
+	w io.Writer,
+	subCmdPrefix []string,
+	subCmdM map[string]subCmd,
+	pM map[string]Param,
+) {
 	type pEntry struct {
 		arg string
 		Param
@@ -200,24 +300,68 @@ func (cli *SourceCLI) printHelp(w io.Writer, pM map[string]Param) {
 		return fmt.Sprint(val.Interface())
 	}
 
-	for _, p := range pA {
-		fmt.Fprintf(w, "\n%s", p.arg)
-		if p.IsBool {
-			fmt.Fprintf(w, " (Flag)")
-		} else if p.Required {
-			fmt.Fprintf(w, " (Required)")
-		} else if defVal := fmtDefaultVal(p.Into); defVal != "" {
-			fmt.Fprintf(w, " (Default: %s)", defVal)
+	type subCmdEntry struct {
+		name string
+		subCmd
+	}
+
+	subCmdA := make([]subCmdEntry, 0, len(subCmdM))
+	for name, subCmd := range subCmdM {
+		if name == "" {
+			name = "<None>"
 		}
-		fmt.Fprintf(w, "\n")
-		if usage := p.Usage; usage != "" {
-			// make all usages end with a period, because I say so
-			usage = strings.TrimSpace(usage)
-			if !strings.HasSuffix(usage, ".") {
-				usage += "."
-			}
-			fmt.Fprintln(w, "\t"+usage)
+		subCmdA = append(subCmdA, subCmdEntry{name: name, subCmd: subCmd})
+	}
+
+	sort.Slice(subCmdA, func(i, j int) bool {
+		return subCmdA[i].name < subCmdA[j].name
+	})
+
+	fmt.Fprintf(w, "Usage: %s", os.Args[0])
+	if len(subCmdPrefix) > 0 {
+		fmt.Fprintf(w, " %s", strings.Join(subCmdPrefix, " "))
+	}
+	if len(subCmdA) > 0 {
+		if _, ok := subCmdM[""]; ok {
+			fmt.Fprint(w, " [sub-command]")
+		} else {
+			fmt.Fprint(w, " <sub-command>")
+		}
+	}
+	if len(pA) > 0 {
+		fmt.Fprint(w, " [options]")
+	}
+	fmt.Fprint(w, "\n\n")
+
+	if len(subCmdA) > 0 {
+		fmt.Fprint(w, "Sub-commands:\n\n")
+		for _, subCmd := range subCmdA {
+			fmt.Fprintf(w, "\t%s\t%s\n", subCmd.name, subCmd.descr)
+		}
+		fmt.Fprint(w, "\n")
+	}
+
+	if len(pA) > 0 {
+		fmt.Fprint(w, "Options:\n\n")
+		for _, p := range pA {
+			fmt.Fprintf(w, "\t%s", p.arg)
+			if p.IsBool {
+				fmt.Fprintf(w, " (Flag)")
+			} else if p.Required {
+				fmt.Fprintf(w, " (Required)")
+			} else if defVal := fmtDefaultVal(p.Into); defVal != "" {
+				fmt.Fprintf(w, " (Default: %s)", defVal)
+			}
+			fmt.Fprint(w, "\n")
+			if usage := p.Usage; usage != "" {
+				// make all usages end with a period, because I say so
+				usage = strings.TrimSpace(usage)
+				if !strings.HasSuffix(usage, ".") {
+					usage += "."
+				}
+				fmt.Fprintln(w, "\t\t"+usage)
+			}
+			fmt.Fprint(w, "\n")
 		}
 	}
-	fmt.Fprintf(w, "\n")
 }
diff --git a/mcfg/cli_test.go b/mcfg/cli_test.go
index e194743..0bdc79f 100644
--- a/mcfg/cli_test.go
+++ b/mcfg/cli_test.go
@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	"regexp"
 	"strings"
 	. "testing"
 	"time"
@@ -16,35 +17,122 @@ import (
 )
 
 func TestSourceCLIHelp(t *T) {
+	assertHelp := func(ctx context.Context, subCmdPrefix []string, exp string) {
+		buf := new(bytes.Buffer)
+		src := &SourceCLI{}
+		pM, err := src.cliParams(CollectParams(ctx))
+		require.NoError(t, err)
+		subCmdM, _ := ctx.Value(cliKeySubCmdM).(map[string]subCmd)
+		src.printHelp(buf, subCmdPrefix, subCmdM, pM)
+
+		out := buf.String()
+		ok := regexp.MustCompile(exp).MatchString(out)
+		assert.True(t, ok, "exp:%s (%q)\ngot:%s (%q)", exp, exp, out, out)
+	}
+
 	ctx := context.Background()
+	assertHelp(ctx, nil, `^Usage: \S+
+
+$`)
+	assertHelp(ctx, []string{"foo", "bar"}, `^Usage: \S+ foo bar
+
+$`)
+
 	ctx, _ = WithInt(ctx, "foo", 5, "Test int param  ") // trailing space should be trimmed
 	ctx, _ = WithBool(ctx, "bar", "Test bool param.")
 	ctx, _ = WithString(ctx, "baz", "baz", "Test string param")
 	ctx, _ = WithRequiredString(ctx, "baz2", "")
 	ctx, _ = WithRequiredString(ctx, "baz3", "")
-	src := SourceCLI{}
 
-	buf := new(bytes.Buffer)
-	pM, err := src.cliParams(collectParams(ctx))
-	require.NoError(t, err)
-	new(SourceCLI).printHelp(buf, pM)
+	assertHelp(ctx, nil, `^Usage: \S+ \[options\]
 
-	exp := `
---baz2 (Required)
+Options:
 
---baz3 (Required)
+	--baz2 \(Required\)
 
---bar (Flag)
-	Test bool param.
+	--baz3 \(Required\)
 
---baz (Default: "baz")
-	Test string param.
+	--bar \(Flag\)
+		Test bool param.
 
---foo (Default: 5)
-	Test int param.
+	--baz \(Default: "baz"\)
+		Test string param.
 
-`
-	assert.Equal(t, exp, buf.String())
+	--foo \(Default: 5\)
+		Test int param.
+
+$`)
+
+	assertHelp(ctx, []string{"foo", "bar"}, `^Usage: \S+ foo bar \[options\]
+
+Options:
+
+	--baz2 \(Required\)
+
+	--baz3 \(Required\)
+
+	--bar \(Flag\)
+		Test bool param.
+
+	--baz \(Default: "baz"\)
+		Test string param.
+
+	--foo \(Default: 5\)
+		Test int param.
+
+$`)
+
+	ctx, _ = WithCLISubCommand(ctx, "first", "First sub-command", nil)
+	ctx, _ = WithCLISubCommand(ctx, "second", "Second sub-command", nil)
+	assertHelp(ctx, []string{"foo", "bar"}, `^Usage: \S+ foo bar <sub-command> \[options\]
+
+Sub-commands:
+
+	first	First sub-command
+	second	Second sub-command
+
+Options:
+
+	--baz2 \(Required\)
+
+	--baz3 \(Required\)
+
+	--bar \(Flag\)
+		Test bool param.
+
+	--baz \(Default: "baz"\)
+		Test string param.
+
+	--foo \(Default: 5\)
+		Test int param.
+
+$`)
+
+	ctx, _ = WithCLISubCommand(ctx, "", "No sub-command", nil)
+	assertHelp(ctx, []string{"foo", "bar"}, `^Usage: \S+ foo bar \[sub-command\] \[options\]
+
+Sub-commands:
+
+	<None>	No sub-command
+	first	First sub-command
+	second	Second sub-command
+
+Options:
+
+	--baz2 \(Required\)
+
+	--baz3 \(Required\)
+
+	--bar \(Flag\)
+		Test bool param.
+
+	--baz \(Default: "baz"\)
+		Test string param.
+
+	--foo \(Default: 5\)
+		Test int param.
+
+$`)
 }
 
 func TestSourceCLI(t *T) {
@@ -164,3 +252,91 @@ func ExampleWithCLITail() {
 	fmt.Printf("err:%v foo:%v bar:%v tail:%#v\n", err, *foo, *bar, *tail)
 	// Output: err:<nil> foo:100 bar:defaultVal tail:[]string{"BADARG", "--bar", "BAR"}
 }
+
+func TestWithCLISubCommand(t *T) {
+	var (
+		ctx         context.Context
+		foo         *int
+		bar         *int
+		baz         *int
+		aFlag       *bool
+		defaultFlag *bool
+	)
+	reset := func() {
+		foo, bar, baz, aFlag, defaultFlag = nil, nil, nil, nil, nil
+		ctx = context.Background()
+		ctx, foo = WithInt(ctx, "foo", 0, "Description of foo.")
+		ctx, aFlag = WithCLISubCommand(ctx, "a", "Description of a.",
+			func(ctx context.Context) context.Context {
+				ctx, bar = WithInt(ctx, "bar", 0, "Description of bar.")
+				return ctx
+			})
+		ctx, defaultFlag = WithCLISubCommand(ctx, "", "Description of default.",
+			func(ctx context.Context) context.Context {
+				ctx, baz = WithInt(ctx, "baz", 0, "Description of baz.")
+				return ctx
+			})
+	}
+
+	reset()
+	_, err := Populate(ctx, &SourceCLI{
+		Args: []string{"a", "--foo=1", "--bar=2"},
+	})
+	massert.Require(t,
+		massert.Comment(massert.Nil(err), "%v", err),
+		massert.Equal(1, *foo),
+		massert.Equal(2, *bar),
+		massert.Nil(baz),
+		massert.Equal(true, *aFlag),
+		massert.Equal(false, *defaultFlag),
+	)
+
+	reset()
+	_, err = Populate(ctx, &SourceCLI{
+		Args: []string{"--foo=1", "--baz=3"},
+	})
+	massert.Require(t,
+		massert.Comment(massert.Nil(err), "%v", err),
+		massert.Equal(1, *foo),
+		massert.Nil(bar),
+		massert.Equal(3, *baz),
+		massert.Equal(false, *aFlag),
+		massert.Equal(true, *defaultFlag),
+	)
+}
+
+func ExampleWithCLISubCommand() {
+	ctx := context.Background()
+	ctx, foo := WithInt(ctx, "foo", 0, "Description of foo.")
+
+	var bar *int
+	ctx, aFlag := WithCLISubCommand(ctx, "a", "Description of a.",
+		func(ctx context.Context) context.Context {
+			ctx, bar = WithInt(ctx, "bar", 0, "Description of bar.")
+			return ctx
+		})
+
+	var baz *int
+	ctx, defaultFlag := WithCLISubCommand(ctx, "", "Description of default.",
+		func(ctx context.Context) context.Context {
+			ctx, baz = WithInt(ctx, "baz", 0, "Description of baz.")
+			return ctx
+		})
+
+	args := []string{"a", "--foo=1", "--bar=2"}
+	if _, err := Populate(ctx, &SourceCLI{Args: args}); err != nil {
+		panic(err)
+	}
+	fmt.Printf("foo:%d bar:%d aFlag:%v defaultFlag:%v\n", *foo, *bar, *aFlag, *defaultFlag)
+
+	// reset output for another Populate
+	*aFlag = false
+	args = []string{"--foo=1", "--baz=3"}
+	if _, err := Populate(ctx, &SourceCLI{Args: args}); err != nil {
+		panic(err)
+	}
+	fmt.Printf("foo:%d baz:%d aFlag:%v defaultFlag:%v\n", *foo, *baz, *aFlag, *defaultFlag)
+
+	// Output: foo:1 bar:2 aFlag:true defaultFlag:false
+	// foo:1 baz:3 aFlag:false defaultFlag:true
+}
diff --git a/mcfg/env.go b/mcfg/env.go
index 8864b98..b94aec6 100644
--- a/mcfg/env.go
+++ b/mcfg/env.go
@@ -43,12 +43,13 @@ func (env *SourceEnv) expectedName(path []string, name string) string {
 }
 
 // Parse implements the method for the Source interface
-func (env *SourceEnv) Parse(ctx context.Context, params []Param) (context.Context, []ParamValue, error) {
+func (env *SourceEnv) Parse(ctx context.Context) (context.Context, []ParamValue, error) {
 	kvs := env.Env
 	if kvs == nil {
 		kvs = os.Environ()
 	}
 
+	params := CollectParams(ctx)
 	pM := map[string]Param{}
 	for _, p := range params {
 		name := env.expectedName(mctx.Path(p.Context), p.Name)
diff --git a/mcfg/mcfg.go b/mcfg/mcfg.go
index 79e63a6..ba570e1 100644
--- a/mcfg/mcfg.go
+++ b/mcfg/mcfg.go
@@ -18,6 +18,12 @@ import (
 // - JSON file
 // - YAML file
 
+// TODO WithCLISubCommand does not play nice with the expected use-case of
+// having CLI params overwrite Env ones. If Env is specified first in the
+// Sources slice then it won't know about any extra Params which might get added
+// due to a sub-command, but if it's specified second then Env values will
+// overwrite CLI ones.
+
 func sortParams(params []Param) {
 	sort.Slice(params, func(i, j int) bool {
 		a, b := params[i], params[j]
@@ -39,10 +45,10 @@ func sortParams(params []Param) {
 	})
 }
 
-// returns all Params gathered by recursively retrieving them from this Context
-// and its children. Returned Params are sorted according to their Path and
-// Name.
-func collectParams(ctx context.Context) []Param {
+// CollectParams returns all Params gathered by recursively retrieving them from
+// this Context and its children. Returned Params are sorted according to their
+// Path and Name.
+func CollectParams(ctx context.Context) []Param {
 	var params []Param
 
 	var visit func(context.Context)
@@ -85,13 +91,18 @@ func paramHash(path []string, name string) string {
 // Source may be nil to indicate that no configuration is provided. Only default
 // values will be used, and if any parameters are required this will error.
 func Populate(ctx context.Context, src Source) (context.Context, error) {
-	params := collectParams(ctx)
 	if src == nil {
 		src = ParamValues(nil)
 	}
 
-	// map Params to their hash, so we can match them to their ParamValues
+	ctx, pvs, err := src.Parse(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	// map Params to their hash, so we can match them to their ParamValues.
 	// later. There should not be any duplicates here.
+	params := CollectParams(ctx)
 	pM := map[string]Param{}
 	for _, p := range params {
 		path := mctx.Path(p.Context)
@@ -102,11 +113,6 @@ func Populate(ctx context.Context, src Source) (context.Context, error) {
 		pM[hash] = p
 	}
 
-	ctx, pvs, err := src.Parse(ctx, params)
-	if err != nil {
-		return nil, err
-	}
-
 	// dedupe the ParamValues based on their hashes, with the last ParamValue
 	// taking precedence. Also filter out those with no corresponding Param.
 	pvM := map[string]ParamValue{}
diff --git a/mcfg/source.go b/mcfg/source.go
index 29f4266..92642cf 100644
--- a/mcfg/source.go
+++ b/mcfg/source.go
@@ -13,14 +13,13 @@ type ParamValue struct {
 	Value json.RawMessage
 }
 
-// Source parses ParamValues out of a particular configuration source, given a
-// sorted set of possible Params to parse, and the Context from with the Params
-// were extracted.
+// Source parses ParamValues out of a particular configuration source, given the
+// Context which the Params were added to (via WithInt, WithString, etc...).
+// CollectParams can be used to retrieve these Params.
 //
 // It's possible for Parsing to affect the Context itself, for example in the
 // case of sub-commands. For this reason Parse can return a Context, which will
-// get used for subsequent Parse commands inside, and then returned from,
-// Populate.
+// get used for subsequent Parse commands inside Populate.
 //
 // Source should not return ParamValues which were not explicitly set to a value
 // by the configuration source.
@@ -30,7 +29,7 @@ type ParamValue struct {
 // ParamValues which do not correspond to any of the passed in Params. These
 // will be ignored in Populate.
 type Source interface {
-	Parse(context.Context, []Param) (context.Context, []ParamValue, error)
+	Parse(context.Context) (context.Context, []ParamValue, error)
 }
 
 // ParamValues is simply a slice of ParamValue elements, which implements Parse
@@ -38,7 +37,7 @@ type Source interface {
 type ParamValues []ParamValue
 
 // Parse implements the method for the Source interface.
-func (pvs ParamValues) Parse(ctx context.Context, _ []Param) (context.Context, []ParamValue, error) {
+func (pvs ParamValues) Parse(ctx context.Context) (context.Context, []ParamValue, error) {
 	return ctx, pvs, nil
 }
 
@@ -48,12 +47,12 @@ func (pvs ParamValues) Parse(ctx context.Context, _ []Param) (context.Context, [
 type Sources []Source
 
 // Parse implements the method for the Source interface.
-func (ss Sources) Parse(ctx context.Context, params []Param) (context.Context, []ParamValue, error) {
+func (ss Sources) Parse(ctx context.Context) (context.Context, []ParamValue, error) {
 	var pvs []ParamValue
 	for _, s := range ss {
 		var innerPVs []ParamValue
 		var err error
-		if ctx, innerPVs, err = s.Parse(ctx, params); err != nil {
+		if ctx, innerPVs, err = s.Parse(ctx); err != nil {
 			return nil, nil, err
 		}
 		pvs = append(pvs, innerPVs...)
diff --git a/mcfg/source_test.go b/mcfg/source_test.go
index 2dafeca..e72483b 100644
--- a/mcfg/source_test.go
+++ b/mcfg/source_test.go
@@ -143,7 +143,7 @@ func (scs srcCommonState) applyCtxAndPV(p srcCommonParams) srcCommonState {
 // ParamValues
 func (scs srcCommonState) assert(s Source) error {
 	root := scs.mkRoot()
-	_, gotPVs, err := s.Parse(root, collectParams(root))
+	_, gotPVs, err := s.Parse(root)
 	if err != nil {
 		return err
 	}
diff --git a/mtest/mtest.go b/mtest/mtest.go
index 6eea8d1..216abed 100644
--- a/mtest/mtest.go
+++ b/mtest/mtest.go
@@ -50,7 +50,8 @@ func Run(ctx context.Context, t *testing.T, body func()) {
 		env = append(env, tup[0]+"="+tup[1])
 	}
 
-	if err := mcfg.Populate(ctx, &mcfg.SourceEnv{Env: env}); err != nil {
+	ctx, err := mcfg.Populate(ctx, &mcfg.SourceEnv{Env: env})
+	if err != nil {
 		t.Fatal(err)
 	} else if err := mrun.Start(ctx); err != nil {
 		t.Fatal(err)