diff --git a/examples/custom/config.yaml b/examples/custom/config.yaml
new file mode 100644
index 0000000..96713bf
--- /dev/null
+++ b/examples/custom/config.yaml
@@ -0,0 +1,7 @@
+app:
+  environment: dev
+
+server:
+  port: 443
+  read_timeout: 1m
+
diff --git a/examples/custom/custom_test.go b/examples/custom/custom_test.go
new file mode 100644
index 0000000..6054d02
--- /dev/null
+++ b/examples/custom/custom_test.go
@@ -0,0 +1,73 @@
+package custom
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/kkyr/fig"
+)
+
+type ListenerType uint
+
+const (
+	ListenerUnix ListenerType = iota
+	ListenerTCP
+	ListenerTLS
+)
+
+type Config struct {
+	App struct {
+		Environment string `fig:"environment" validate:"required"`
+	} `fig:"app"`
+	Server struct {
+		Host     string       `fig:"host" default:"0.0.0.0"`
+		Port     int          `fig:"port" default:"80"`
+		Listener ListenerType `fig:"listener_type" default:"tcp"`
+	} `fig:"server"`
+}
+
+func ExampleLoad() {
+	var cfg Config
+	err := fig.Load(&cfg)
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Println(cfg.App.Environment)
+	fmt.Println(cfg.Server.Host)
+	fmt.Println(cfg.Server.Port)
+	fmt.Println(cfg.Server.Listener)
+
+	// Output:
+	// dev
+	// 0.0.0.0
+	// 443
+	// tcp
+}
+
+func (l *ListenerType) UnmarshalString(v string) error {
+	switch strings.ToLower(v) {
+	case "unix":
+		*l = ListenerUnix
+	case "tcp":
+		*l = ListenerTCP
+	case "tls":
+		*l = ListenerTLS
+	default:
+		return fmt.Errorf("unknown listener type: %s", v)
+	}
+	return nil
+}
+
+func (l ListenerType) String() string {
+	switch l {
+	case ListenerUnix:
+		return "unix"
+	case ListenerTCP:
+		return "tcp"
+	case ListenerTLS:
+		return "tls"
+	default:
+		return "unknown"
+	}
+}
diff --git a/fig.go b/fig.go
index 4f6232a..3da9d60 100644
--- a/fig.go
+++ b/fig.go
@@ -28,6 +28,42 @@ const (
 	DefaultTimeLayout = time.RFC3339
 )
 
+// StringUnmarshaler is an interface for custom unmarshaling of strings
+//
+// If a field with a local type asignment satisfies this interface, it allows the user
+// to implment their own custom type unmarshaling method.
+//
+// Example:
+//
+//	type ListenerType uint
+//
+//	const (
+//	  ListenerUnix ListenerType = iota
+//	  ListenerTCP
+//	  ListenerTLS
+//	)
+//
+//	type Config struct {
+//	  Listener ListenerType `fig:"listener_type" default:"unix"`
+//	}
+//
+//	func (l *ListenerType) UnmarshalType(v string) error {
+//		   switch strings.ToLower(v) {
+//		   case "unix":
+//		     *l = ListenerUnix
+//		   case "tcp":
+//		     *l = ListenerTCP
+//		   case "tls":
+//		     *l = ListenerTLS
+//		   default:
+//	      return fmt.Errorf("unknown listener type: %s", v)
+//	    }
+//	    return nil
+//	 }
+type StringUnmarshaler interface {
+	UnmarshalString(s string) error
+}
+
 // Load reads a configuration file and loads it into the given struct. The
 // parameter `cfg` must be a pointer to a struct.
 //
@@ -158,6 +194,7 @@ func (f *fig) decodeMap(m map[string]interface{}, result interface{}) error {
 			mapstructure.StringToTimeDurationHookFunc(),
 			mapstructure.StringToTimeHookFunc(f.timeLayout),
 			stringToRegexpHookFunc(),
+			stringToStringUnmarshalerHook(),
 		),
 	})
 	if err != nil {
@@ -183,6 +220,36 @@ func stringToRegexpHookFunc() mapstructure.DecodeHookFunc {
 	}
 }
 
+// stringToStringUnmarshalerHook returns a DecodeHookFunc that executes a custom method which
+// satisfies the StringUnmarshaler interface on custom types.
+func stringToStringUnmarshalerHook() mapstructure.DecodeHookFunc {
+	return func(f reflect.Type, t reflect.Type, data interface{}) (interface{}, error) {
+		if f.Kind() != reflect.String {
+			return data, nil
+		}
+
+		ds, ok := data.(string)
+		if !ok {
+			return data, nil
+		}
+
+		if reflect.PointerTo(t).Implements(reflect.TypeOf((*StringUnmarshaler)(nil)).Elem()) {
+			val := reflect.New(t).Interface()
+
+			if unmarshaler, ok := val.(StringUnmarshaler); ok {
+				err := unmarshaler.UnmarshalString(ds)
+				if err != nil {
+					return nil, err
+				}
+
+				return reflect.ValueOf(val).Elem().Interface(), nil
+			}
+		}
+
+		return data, nil
+	}
+}
+
 // processCfg processes a cfg struct after it has been loaded from
 // the config file, by validating required fields and setting defaults
 // where applicable.
@@ -257,9 +324,24 @@ func (f *fig) setDefaultValue(fv reflect.Value, val string) error {
 
 // setValue sets fv to val. it attempts to convert val to the correct
 // type based on the field's kind. if conversion fails an error is
-// returned.
+// returned. If fv satisfies the StringUnmarshaler interface it will
+// execute the corresponding StringUnmarshaler.UnmarshalString method
+// on the value.
 // fv must be settable else this panics.
 func (f *fig) setValue(fv reflect.Value, val string) error {
+	if fv.IsValid() && reflect.PointerTo(fv.Type()).Implements(reflect.TypeOf((*StringUnmarshaler)(nil)).Elem()) {
+		vi := reflect.New(fv.Type()).Interface()
+		if unmarshaler, ok := vi.(StringUnmarshaler); ok {
+			err := unmarshaler.UnmarshalString(val)
+			if err != nil {
+				return fmt.Errorf("could not unmarshal string %q: %w", val, err)
+			}
+			fv.Set(reflect.ValueOf(vi).Elem())
+			return nil
+		}
+		return fmt.Errorf("unexpected error while trying to unmarshal string")
+	}
+
 	switch fv.Kind() {
 	case reflect.Ptr:
 		if fv.IsNil() {
diff --git a/fig_test.go b/fig_test.go
index 737ab64..36e0573 100644
--- a/fig_test.go
+++ b/fig_test.go
@@ -84,6 +84,28 @@ type Item struct {
 	Path string `fig:"path" validate:"required"`
 }
 
+type ListenerType uint
+
+const (
+	ListenerUnix ListenerType = iota
+	ListenerTCP
+	ListenerTLS
+)
+
+func (l *ListenerType) UnmarshalString(v string) error {
+	switch strings.ToLower(v) {
+	case "unix":
+		*l = ListenerUnix
+	case "tcp":
+		*l = ListenerTCP
+	case "tls":
+		*l = ListenerTLS
+	default:
+		return fmt.Errorf("unknown listener type: %s", v)
+	}
+	return nil
+}
+
 func validPodConfig() Pod {
 	var pod Pod
 
@@ -249,6 +271,7 @@ func Test_fig_Load_Defaults(t *testing.T) {
 					Application struct {
 						BuildDate time.Time `fig:"build_date" default:"2020-01-01T12:00:00Z"`
 					}
+					Listener ListenerType `fig:"listener_type" default:"unix"`
 				}
 
 				var want Server
@@ -259,6 +282,7 @@ func Test_fig_Load_Defaults(t *testing.T) {
 				want.Logger.Production = false
 				want.Logger.Metadata.Keys = []string{"ts"}
 				want.Application.BuildDate = time.Date(2020, 1, 1, 12, 0, 0, 0, time.UTC)
+				want.Listener = ListenerUnix
 
 				var cfg Server
 				err := Load(&cfg, File(f), Dirs(filepath.Join("testdata", "valid")))
@@ -590,8 +614,9 @@ func Test_fig_decodeMap(t *testing.T) {
 		"log_level": "debug",
 		"severity":  "5",
 		"server": map[string]interface{}{
-			"ports":  []int{443, 80},
-			"secure": 1,
+			"ports":         []int{443, 80},
+			"secure":        1,
+			"listener_type": "tls",
 		},
 	}
 
@@ -599,8 +624,9 @@ func Test_fig_decodeMap(t *testing.T) {
 		Level    string `fig:"log_level"`
 		Severity int    `fig:"severity" validate:"required"`
 		Server   struct {
-			Ports  []string `fig:"ports" default:"[443]"`
-			Secure bool
+			Ports    []string `fig:"ports" default:"[443]"`
+			Secure   bool
+			Listener ListenerType `fig:"listener_type" default:"unix"`
 		} `fig:"server"`
 	}
 
@@ -623,6 +649,10 @@ func Test_fig_decodeMap(t *testing.T) {
 	if cfg.Server.Secure == false {
 		t.Error("cfg.Server.Secure == false")
 	}
+
+	if cfg.Server.Listener != ListenerTLS {
+		t.Errorf("cfg.Server.Listener: want: %d, got: %d", ListenerTLS, cfg.Server.Listener)
+	}
 }
 
 func Test_fig_processCfg(t *testing.T) {
diff --git a/testdata/valid/server.toml b/testdata/valid/server.toml
index a5ba440..ad60113 100644
--- a/testdata/valid/server.toml
+++ b/testdata/valid/server.toml
@@ -1,4 +1,4 @@
 host = "0.0.0.0"
 
 [logger]
-log_level = "debug"
\ No newline at end of file
+log_level = "debug"