diff --git a/go.mod b/go.mod
index 0abdf6f..ac857c8 100644
--- a/go.mod
+++ b/go.mod
@@ -3,13 +3,13 @@ module github.com/ucan-wg/go-ucan
 go 1.15
 
 require (
-	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/golang-jwt/jwt v3.2.2+incompatible
 	github.com/ipfs/go-cid v0.0.7
 	github.com/libp2p/go-libp2p-core v0.7.0
 	github.com/multiformats/go-multibase v0.0.3
 	github.com/multiformats/go-multihash v0.0.14
 	github.com/multiformats/go-varint v0.0.6
+	github.com/stretchr/testify v1.9.0
 	golang.org/x/crypto v0.0.0-20210322153248-0c34fe9e7dc2 // indirect
 	golang.org/x/sys v0.0.0-20210403161142-5e06dd20ab57 // indirect
 )
diff --git a/go.sum b/go.sum
index bad317a..1636311 100644
--- a/go.sum
+++ b/go.sum
@@ -71,13 +71,22 @@ github.com/multiformats/go-varint v0.0.6/go.mod h1:3Ls8CIEsrijN6+B7PbrXRPxHRPuXS
 github.com/onsi/ginkgo v1.6.0/go.mod h1:lLunBs/Ym6LB5Z9jYTR76FiuTmxDTDusOGeTQH+WWjE=
 github.com/onsi/ginkgo v1.7.0/go.mod h1:lLunBs/Ym6LB5Z9jYTR76FiuTmxDTDusOGeTQH+WWjE=
 github.com/onsi/gomega v1.4.3/go.mod h1:ex+gbHU/CVuBBDIJjb2X0qEXbFg53c61hWP/1CpauHY=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/spacemonkeygo/spacelog v0.0.0-20180420211403-2296661a0572 h1:RC6RW7j+1+HkWaX/Yh71Ee5ZHaHYt7ZP4sQgUrm6cDU=
 github.com/spacemonkeygo/spacelog v0.0.0-20180420211403-2296661a0572/go.mod h1:w0SWMsp6j9O/dk4/ZpIhL+3CkG8ofA2vuv7k+ltqUMc=
 github.com/spaolacci/murmur3 v1.1.0 h1:7c1g84S4BPRrfL5Xrdp6fOJ206sU9y293DDHaoy0bLI=
 github.com/spaolacci/murmur3 v1.1.0/go.mod h1:JwIasOWyU6f++ZhiEuf87xNszmSA2myDM2Kzu9HwQUA=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/objx v0.4.0/go.mod h1:YvHI0jy2hoMjB+UWwv71VJQ9isScKT/TqJzVSSt89Yw=
+github.com/stretchr/objx v0.5.0/go.mod h1:Yh+to48EsGEfYuaHDzXPcE3xhTkx73EhmCGUpEOglKo=
+github.com/stretchr/objx v0.5.2/go.mod h1:FRsXN1f5AsAjCGJKqEizvkpNtU+EGNCLh3NxZ/8L+MA=
 github.com/stretchr/testify v1.4.0/go.mod h1:j7eGeouHqKxXV5pUuKE4zz7dFj8WfuZ+81PSLYec5m4=
+github.com/stretchr/testify v1.7.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
+github.com/stretchr/testify v1.8.0/go.mod h1:yNjHg4UonilssWZ8iaSj1OCr/vHnekPRkoO+kdMU+MU=
+github.com/stretchr/testify v1.8.4/go.mod h1:sz/lmYIOXD/1dqDmKjjqLyZ2RngseejIcXlSw2iwfAo=
+github.com/stretchr/testify v1.9.0 h1:HtqpIVDClZ4nwg75+f6Lvsy/wHu+3BoSGCbBAcpTsTg=
+github.com/stretchr/testify v1.9.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
 go.opencensus.io v0.22.4/go.mod h1:yxeiOL68Rb0Xd1ddK5vPZ/oVn4vY4Ynel7k9FzqtOIw=
 golang.org/x/crypto v0.0.0-20170930174604-9419663f5a44/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
@@ -124,9 +133,13 @@ google.golang.org/genproto v0.0.0-20180817151627-c66870c02cf8/go.mod h1:JiN7NxoA
 google.golang.org/genproto v0.0.0-20190425155659-357c62f0e4bb/go.mod h1:VzzqZJRnGkLBvHegQrXjBqPurQTc5/KpmUdxsrq26oE=
 google.golang.org/grpc v1.19.0/go.mod h1:mqu4LbDTu4XGKhr4mRzUsmM4RtVoemTSY81AxZiDr8c=
 google.golang.org/grpc v1.20.1/go.mod h1:10oTOabMzJvdu6/UiuZezV6QK5dSlG84ov/aaiqXj38=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/fsnotify.v1 v1.4.7/go.mod h1:Tz8NjZHkW78fSQdbUxIjBTcgA1z1m8ZHf0WmKUhAMys=
 gopkg.in/tomb.v1 v1.0.0-20141024135613-dd632973f1e7/go.mod h1:dt/ZhP58zS4L8KSrWDmTeBkI65Dw0HsyUHuEVlX15mw=
 gopkg.in/yaml.v2 v2.2.1/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 honnef.co/go/tools v0.0.0-20190102054323-c2f93a96b099/go.mod h1:rf3lG4BRIbNafJWhAfAdb/ePZxsR/4RtNHQocxwk9r4=
diff --git a/pkg/command/command.go b/pkg/command/command.go
new file mode 100644
index 0000000..3c51416
--- /dev/null
+++ b/pkg/command/command.go
@@ -0,0 +1,142 @@
+package command
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+)
+
+const (
+	separator = "/"
+)
+
+// ErrNew indicates that the wrapped error was encountered while creating
+// a new Command.
+var ErrNew = errors.New("failed to create Command from elems")
+
+// ErrParse indicates that the wrapped error was encountered while
+// attempting to parse a string as a Command.
+var ErrParse = errors.New("failed to parse Command")
+
+// ErrorJoin indicates that the wrapped error was encountered while
+// attempting to join a new segment to a Command.
+var ErrJoin = errors.New("failed to join segments to Command")
+
+// ErrRequiresLeadingSlash is returned when a parsing a string that
+// doesn't start with a [leading slash character].
+//
+// [leading slash character]: https://github.com/ucan-wg/spec#segment-structure
+var ErrRequiresLeadingSlash = parseError("a command requires a leading slash character")
+
+// ErrDisallowsTrailingSlash is returned when parsing a string that [ends
+// with a trailing slash character].
+//
+// [ends with a trailing slash character]: https://github.com/ucan-wg/spec#segment-structure
+var ErrDisallowsTrailingSlash = parseError("a command must not include a trailing slash")
+
+// ErrUCANNamespaceReserved is returned to indicate that a Command's
+// first segment would contain the [reserved "ucan" namespace].
+//
+// [reserved "ucan" namespace]: https://github.com/ucan-wg/spec#ucan-namespace
+var ErrUCANNamespaceReserved = errors.New("the UCAN namespace is reserved")
+
+// ErrRequiresLowercase is returned if a Command contains, or would contain,
+// [uppercase unicode characters].
+//
+// [uppercase unicode characters]: https://github.com/ucan-wg/spec#segment-structure
+var ErrRequiresLowercase = parseError("UCAN path segments must must not contain upper-case characters")
+
+func parseError(msg string) error {
+	return fmt.Errorf("%w: %s", ErrParse, msg)
+}
+
+var _ fmt.Stringer = (*Command)(nil)
+
+// Command is a concrete messages (a "verb") that MUST be unambiguously
+// interpretable by the Subject of a UCAN.
+//
+// A [Command] is composed of a leading slash which is optionally followed
+// by one or more slash-separated Segments of lowercase characters.
+//
+// [Command]: https://github.com/ucan-wg/spec#command
+type Command struct {
+	segments []string
+}
+
+// New creates a validated command from the provided list of segment
+// strings.  An error is returned if an invalid Command would be
+// formed
+func New(segments ...string) (*Command, error) {
+	return newCommand(ErrNew, segments...)
+}
+
+func newCommand(err error, segments ...string) (*Command, error) {
+	if len(segments) > 0 && segments[0] == "ucan" {
+		return nil, fmt.Errorf("%w: %w", err, ErrUCANNamespaceReserved)
+	}
+
+	cmd := Command{segments}
+
+	return &cmd, nil
+}
+
+// Parse verifies that the provided string contains the required
+// [segment structure] and, if valid, returns the resulting
+// Command.
+//
+// [segment structure]: https://github.com/ucan-wg/spec#segment-structure
+func Parse(s string) (*Command, error) {
+	if !strings.HasPrefix(s, "/") {
+		return nil, ErrRequiresLeadingSlash
+	}
+
+	if len(s) > 1 && strings.HasSuffix(s, "/") {
+		return nil, ErrDisallowsTrailingSlash
+	}
+
+	if s != strings.ToLower(s) {
+		return nil, ErrRequiresLowercase
+	}
+
+	// The leading slash will result in the first element from strings.Split
+	// being an empty string which is removed as strings.Join will ignore it.
+	return newCommand(ErrParse, strings.Split(s, "/")[1:]...)
+}
+
+// [Top] is the most powerful capability.
+//
+// This function returns a Command that is a wildcard and therefore represents the
+// most powerful abilily.  As such it should be handle with care and used
+// sparingly.
+//
+// [Top]: https://github.com/ucan-wg/spec#-aka-top
+func Top() *Command {
+	cmd, _ := New()
+
+	return cmd
+}
+
+// IsValid returns true if the provided string is a valid UCAN command.
+func IsValid(s string) bool {
+	_, err := Parse(s)
+
+	return err == nil
+}
+
+// Join appends segments to the end of this command using the required
+// segment separator.
+func (c *Command) Join(segments ...string) (*Command, error) {
+	return newCommand(ErrJoin, append(c.segments, segments...)...)
+}
+
+// Segments returns the ordered segments that comprise the Command as a
+// slice of strings.
+func (c *Command) Segments() []string {
+	return c.segments
+}
+
+// String returns the composed representation the command.  This is also
+// the required wire representation (before IPLD encoding occurs.)
+func (c *Command) String() string {
+	return "/" + strings.Join([]string(c.segments), "/")
+}
diff --git a/pkg/command/command_test.go b/pkg/command/command_test.go
new file mode 100644
index 0000000..7d83004
--- /dev/null
+++ b/pkg/command/command_test.go
@@ -0,0 +1,159 @@
+package command_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/ucan-wg/go-ucan/pkg/command"
+)
+
+func TestCommand_String(t *testing.T) {
+	require.Equal(t, "/", command.Top().String())
+}
+
+func TestIsValidCommand(t *testing.T) {
+	t.Parallel()
+
+	t.Run("succeeds when", func(t *testing.T) {
+		t.Parallel()
+
+		for _, testcase := range validTestcases(t) {
+			testcase := testcase
+
+			t.Run(testcase.name, func(t *testing.T) {
+				t.Parallel()
+
+				require.True(t, command.IsValid(testcase.inp))
+			})
+		}
+	})
+
+	t.Run("fails when", func(t *testing.T) {
+		t.Parallel()
+
+		for _, testcase := range invalidTestcases(t) {
+			testcase := testcase
+
+			t.Run(testcase.name, func(t *testing.T) {
+				t.Parallel()
+
+				require.False(t, command.IsValid(testcase.inp))
+			})
+		}
+	})
+}
+
+func TestParseCommand(t *testing.T) {
+	t.Parallel()
+
+	t.Run("succeeds when", func(t *testing.T) {
+		t.Parallel()
+
+		for _, testcase := range validTestcases(t) {
+			testcase := testcase
+
+			t.Run(testcase.name, func(t *testing.T) {
+				t.Parallel()
+
+				cmd, err := command.Parse("/elem0/elem1/elem2")
+				require.NoError(t, err)
+				require.NotNil(t, cmd)
+			})
+		}
+	})
+
+	t.Run("fails when", func(t *testing.T) {
+		t.Parallel()
+
+		for _, testcase := range invalidTestcases(t) {
+			testcase := testcase
+
+			t.Run(testcase.name, func(t *testing.T) {
+				t.Parallel()
+
+				cmd, err := command.Parse(testcase.inp)
+				assert.ErrorIs(t, err, command.ErrParse)
+				assert.ErrorIs(t, err, testcase.err)
+				assert.Nil(t, cmd)
+			})
+		}
+	})
+}
+
+type testcase struct {
+	name string
+	inp  string
+}
+
+func validTestcases(t *testing.T) []testcase {
+	t.Helper()
+
+	cmds := []string{
+		"/",
+		"/crud",
+		"/crud/create",
+		"/stack/pop",
+		"/crypto/sign",
+		"/foo/bar/baz/qux/quux",
+		"/ほげ/ふが",
+	}
+
+	testcases := make([]testcase, len(cmds))
+
+	for i, inp := range cmds {
+		testcases[i] = testcase{
+			name: "Command is " + inp,
+			inp:  inp,
+		}
+	}
+
+	return testcases
+}
+
+type errorTestcase struct {
+	testcase
+	err error
+}
+
+func invalidTestcases(t *testing.T) []errorTestcase {
+	t.Helper()
+
+	return []errorTestcase{
+		{
+			testcase: testcase{
+				name: "leading slash is missing",
+				inp:  "elem0/elem1/elem2",
+			},
+			err: command.ErrRequiresLeadingSlash,
+		},
+		{
+			testcase: testcase{
+				name: "trailing slash is present",
+				inp:  "/elem0/elem1/elem2/",
+			},
+			err: command.ErrDisallowsTrailingSlash,
+		},
+		{
+			testcase: testcase{
+				name: "only reserved ucan namespace",
+				inp:  "/ucan",
+			},
+			err: command.ErrUCANNamespaceReserved,
+		},
+		{
+			testcase: testcase{
+				name: "reserved ucan namespace prefix",
+				inp:  "/ucan/elem0/elem1/elem2",
+			},
+			err: command.ErrUCANNamespaceReserved,
+		},
+		{
+			testcase: testcase{
+				name: "uppercase character are present",
+				inp:  "/elem0/Elem1/elem2",
+			},
+			err: command.ErrRequiresLowercase,
+		},
+	}
+}