diff --git a/environment.go b/environment.go
index 846b846..e25dfbb 100644
--- a/environment.go
+++ b/environment.go
@@ -16,21 +16,28 @@ package environment
 
 import (
 	"sync"
+	"sync/atomic"
 )
 
 var (
 	// Mutex when operating on the current runtime environment.
-	mutex sync.Mutex
+	mutex sync.RWMutex
 
 	// The current environment.
 	current = Development
 
+	// Is the current runtime environment locked?
+	locked = int32(0)
+
 	// List of supported environments.
 	supported = []Env{Development, Testing, Prerelease, Production}
 )
 
 // Get returns the current runtime environment.
 func Get() Env {
+	mutex.RLock()
+	defer mutex.RUnlock()
+
 	return current
 }
 
@@ -46,12 +53,34 @@ func Register(env Env) {
 	}
 }
 
+// Lock locks the current runtime environment.
+// After locking, the current runtime environment cannot be changed.
+func Lock() {
+	mutex.Lock()
+	defer mutex.Unlock()
+
+	atomic.StoreInt32(&locked, 1)
+}
+
+// Locked returns whether the current runtime environment is locked.
+func Locked() bool {
+	return atomic.LoadInt32(&locked) == 1
+}
+
 // Set sets the current runtime environment.
-// If the given environment is not supported, an ErrInvalidEnv error is returned.
+// If the given runtime environment is not supported, ErrInvalidEnv error is returned.
+// If the current runtime environment is locked, ErrLocked error is returned.
 func Set(env Env) error {
 	mutex.Lock()
 	defer mutex.Unlock()
 
+	return doSet(env)
+}
+
+func doSet(env Env) error {
+	if Locked() {
+		return ErrLocked
+	}
 	if !env.In(supported) {
 		return ErrInvalidEnv
 	}
@@ -59,3 +88,17 @@ func Set(env Env) error {
 	current = env
 	return nil
 }
+
+// SetAndLock sets and locks the current runtime environment.
+// If the runtime environment settings fail, they are not locked.
+func SetAndLock(env Env) error {
+	mutex.Lock()
+	defer mutex.Unlock()
+
+	if err := doSet(env); err != nil {
+		return err
+	}
+
+	atomic.StoreInt32(&locked, 1)
+	return nil
+}
diff --git a/environment_test.go b/environment_test.go
index 57e0558..1763b7a 100644
--- a/environment_test.go
+++ b/environment_test.go
@@ -15,36 +15,109 @@
 package environment
 
 import (
+	"sync/atomic"
 	"testing"
 )
 
-func TestEnvironment(t *testing.T) {
+func doTestEnvironment(f func()) {
+	// Oh, we still need to restore the scene!
 	defer func() {
 		current = Development
+		atomic.StoreInt32(&locked, 0)
 		supported = []Env{Development, Testing, Prerelease, Production}
 	}()
 
-	if got := Get(); got != Development {
-		t.Fatal(got)
-	}
+	f()
+}
+
+func TestEnvironment(t *testing.T) {
+	// Copy from supported.
+	list := []Env{Development, Testing, Prerelease, Production}
 
-	envs := []Env{Development, Testing, Prerelease, Production}
-	for i, j := 0, len(envs); i < j; i++ {
-		if err := Set(envs[i]); err != nil {
+	doTestEnvironment(func() {
+		// The default runtime environment is Development!
+		if got := Get(); got != Development {
+			t.Fatal(got)
+		}
+
+		for _, env := range list {
+			if err := Set(env); err != nil {
+				t.Fatal(err)
+			}
+			if got := Get(); got != env {
+				t.Fatal(got)
+			}
+		}
+
+		if err := Set("foo"); err == nil {
+			t.Fatal("No error")
+		} else {
+			if err != ErrInvalidEnv {
+				t.Fatal(err)
+			}
+		}
+
+		Register("foo")
+
+		if err := Set("foo"); err != nil {
 			t.Fatal(err)
 		}
-	}
 
-	if err := Set("foo"); err == nil {
-		t.Fatal(`Set("foo")`)
-	} else {
-		if err != ErrInvalidEnv {
+		if Locked() {
+			t.Fatal("Locked")
+		}
+
+		Lock()
+
+		if !Locked() {
+			t.Fatal("Not Locked")
+		}
+		for _, env := range list {
+			if err := Set(env); err == nil {
+				t.Fatal("No error")
+			} else {
+				if err != ErrLocked {
+					t.Fatal(err)
+				}
+			}
+		}
+		if err := Set("foo"); err == nil {
+			t.Fatal("No error")
+		} else {
+			if err != ErrLocked {
+				t.Fatal(err)
+			}
+		}
+	})
+}
+
+func TestSetAndLock(t *testing.T) {
+	doTestEnvironment(func() {
+		if Locked() {
+			t.Fatal("Locked")
+		}
+
+		// The default runtime environment is Development!
+		if got := Get(); got != Development {
+			t.Fatal(got)
+		}
+
+		if err := SetAndLock(Testing); err != nil {
 			t.Fatal(err)
 		}
-	}
+		if !Locked() {
+			t.Fatal("Not Locked")
+		}
+		if got := Get(); got != Testing {
+			t.Fatal(got)
+		}
 
-	Register("foo")
-	if err := Set("foo"); err != nil {
-		t.Fatal(err)
-	}
+		if err := SetAndLock(Production); err == nil {
+			t.Fatal("No error")
+		} else {
+			if err != ErrLocked {
+				t.Fatal(err)
+			}
+		}
+	})
 }
diff --git a/errors.go b/errors.go
index 8f2281a..51941b7 100644
--- a/errors.go
+++ b/errors.go
@@ -20,4 +20,8 @@ import (
 
 // ErrInvalidEnv represents that the given runtime environment is not
 // registered or supported.
-var ErrInvalidEnv = errors.New("invalid environment")
+var ErrInvalidEnv = errors.New("invalid runtime environment")
+
+// ErrLocked indicates that the current runtime environment is locked
+// and cannot be changed.
+var ErrLocked = errors.New("locked runtime environment")