diff --git a/pkg/api/api.go b/pkg/api/api.go
index 1b449c3..a03fd07 100644
--- a/pkg/api/api.go
+++ b/pkg/api/api.go
@@ -31,6 +31,7 @@ type vcsPath struct {
 	vcs    func(module string) vcs.VCS
 }
 
+// Option configures an API handler.
 type Option func(*api)
 
 var (
@@ -40,6 +41,7 @@ var (
 	apiZip  = regexp.MustCompile(`^/(?P<module>.*)/@v/(?P<version>.*).zip$`)
 )
 
+// New returns a configured http.Handler which implements GOPROXY API.
 func New(options ...Option) http.Handler {
 	api := &api{log: func(...interface{}) {}}
 	for _, opt := range options {
@@ -48,8 +50,13 @@ func New(options ...Option) http.Handler {
 	return api
 }
 
+// Log configures API to use a specific logger function, such as log.Println,
+// testing.T.Log or any other custom logger.
 func Log(log logger) Option { return func(api *api) { api.log = log } }
 
+// Git configures API to use a specific git client when trying to download a
+// repository with the given prefix. Auth string can be a path to the SSK key,
+// or a colon-separated username:password string.
 func Git(prefix string, auth string) Option {
 	a := vcs.Key(auth)
 	if creds := strings.SplitN(auth, ":", 2); len(creds) == 2 {
@@ -65,12 +72,14 @@ func Git(prefix string, auth string) Option {
 	}
 }
 
+// Memory configures API to use in-memory cache for downloaded modules.
 func Memory() Option {
 	return func(api *api) {
 		api.stores = append(api.stores, store.Memory())
 	}
 }
 
+// CacheDir configures API to use a local disk storage for downloaded modules.
 func CacheDir(dir string) Option {
 	return func(api *api) {
 		api.stores = append(api.stores, store.Disk(dir))
diff --git a/pkg/store/disk.go b/pkg/store/disk.go
index 0a4610f..a5d4926 100644
--- a/pkg/store/disk.go
+++ b/pkg/store/disk.go
@@ -11,6 +11,7 @@ import (
 
 type disk string
 
+// Disk returns a local disk cache that stores files within a given directory.
 func Disk(dir string) Store { return disk(dir) }
 
 func (d disk) Put(ctx context.Context, snapshot Snapshot) error {
diff --git a/pkg/store/mem.go b/pkg/store/mem.go
index 3efc42e..7a0983f 100644
--- a/pkg/store/mem.go
+++ b/pkg/store/mem.go
@@ -13,6 +13,7 @@ type memory struct {
 	cache []Snapshot
 }
 
+// Memory creates an in-memory cache.
 func Memory() Store { return &memory{} }
 
 func (m *memory) Put(ctx context.Context, snapshot Snapshot) error {
diff --git a/pkg/store/store.go b/pkg/store/store.go
index 6df5e35..7c06729 100644
--- a/pkg/store/store.go
+++ b/pkg/store/store.go
@@ -7,12 +7,15 @@ import (
 	"github.com/sixt/gomodproxy/pkg/vcs"
 )
 
+// Store is an interface for a typical cache. It allows to put a snapshot and
+// to get snapshot of the specific version.
 type Store interface {
 	Put(ctx context.Context, snapshot Snapshot) error
 	Get(ctx context.Context, module string, version vcs.Version) (Snapshot, error)
 	Close() error
 }
 
+// Snapshot is a module source code of the speciic version.
 type Snapshot struct {
 	Module    string
 	Version   vcs.Version
@@ -20,6 +23,7 @@ type Snapshot struct {
 	Data      []byte
 }
 
+// Key returns a snapshot key string that can be used in cache stores.
 func (s Snapshot) Key() string {
 	return s.Module + "@" + string(s.Version)
 }
diff --git a/pkg/vcs/git.go b/pkg/vcs/git.go
index 2510a24..9f26ba4 100644
--- a/pkg/vcs/git.go
+++ b/pkg/vcs/git.go
@@ -30,6 +30,8 @@ type gitVCS struct {
 	auth   Auth
 }
 
+// NewGit return a go-git VCS client implementation that provides information
+// about the specific module using the pgiven authentication mechanism.
 func NewGit(l logger, module string, auth Auth) VCS {
 	return &gitVCS{log: l, module: module, auth: auth}
 }
diff --git a/pkg/vcs/vcs.go b/pkg/vcs/vcs.go
index 77a643e..fd04700 100644
--- a/pkg/vcs/vcs.go
+++ b/pkg/vcs/vcs.go
@@ -13,11 +13,15 @@ import (
 
 type logger = func(v ...interface{})
 
+// Version represents a semantic version of a module.
 type Version string
 
 var reSemVer = regexp.MustCompile(`^v\d+\.\d+\.\d+$`)
 
+// IsSemVer returns true if a version string is a semantic version e.g. vX.Y.Z.
 func (v Version) IsSemVer() bool { return reSemVer.MatchString(string(v)) }
+
+// Hash returns a commit hash if a version is of a form v0.0.0-timestamp-hash.
 func (v Version) Hash() string {
 	fields := strings.Split(string(v), "-")
 	if len(fields) != 3 {
@@ -26,26 +30,38 @@ func (v Version) Hash() string {
 	return fields[2]
 }
 
+// Module is a source code snapshot for which one can get the commit timestamp
+// or the actual ZIP with the source code in it.
 type Module interface {
 	Timestamp(ctx context.Context, version Version) (time.Time, error)
 	Zip(ctx context.Context, version Version) (io.ReadCloser, error)
 }
 
+// VCS is a version control system client. It can list available versions from
+// the remote, as well as fetch module data such as timestamp or zip snapshot.
 type VCS interface {
 	List(ctx context.Context) ([]Version, error)
 	Module
 }
 
+// Auth defines a typical VCS authentication mechanism, such as SSH key or
+// username/password.
 type Auth struct {
 	Username string
 	Password string
 	Key      string
 }
 
-func NoAuth() Auth                            { return Auth{} }
-func Password(username, password string) Auth { return Auth{Username: username, Password: password} }
-func Key(key string) Auth                     { return Auth{Key: key} }
+// NoAuth returns an Auth implementation that uses no authentication at all.
+func NoAuth() Auth { return Auth{} }
 
+// Password returns an Auth implementation that authenticate via username and password.
+func Password(username, password string) Auth { return Auth{Username: username, Password: password} }
+
+// Key returns an Auth implementation that uses key file authentication mechanism.
+func Key(key string) Auth { return Auth{Key: key} }
+
+// MetaImports resolved module import path for certain hosts using the special <meta> tag.
 func MetaImports(ctx context.Context, module string) (string, error) {
 	if strings.HasPrefix(module, "github.com/") || strings.HasPrefix(module, "bitbucket.org/") {
 		return module, nil