diff --git a/pkg/api/v1beta1/types.go b/pkg/api/v1beta1/types.go
index abe22aee838..6b2a2dc4765 100644
--- a/pkg/api/v1beta1/types.go
+++ b/pkg/api/v1beta1/types.go
@@ -158,7 +158,8 @@ type ExecProbe struct {
 	// Command is the command line to execute inside the container, the working directory for the
 	// command  is root ('/') in the container's filesystem.  The command is simply exec'd, it is
 	// not run inside a shell, so traditional shell instructions ('|', etc) won't work.  To use
-	// a shell, you need to explicitly call out to that shell
+	// a shell, you need to explicitly call out to that shell.
+	// A return code of zero is treated as 'Healthy', non-zero is 'Unhealthy'
 	Command []string `yaml:"command,omitempty" json:"command,omitempty"`
 }
 
diff --git a/pkg/health/health.go b/pkg/health/health.go
index 5922f562a4a..637250157d8 100644
--- a/pkg/health/health.go
+++ b/pkg/health/health.go
@@ -17,6 +17,8 @@ limitations under the License.
 package health
 
 import (
+	"sync"
+
 	"github.com/GoogleCloudPlatform/kubernetes/pkg/api"
 	"github.com/golang/glog"
 )
@@ -36,16 +38,32 @@ type HealthChecker interface {
 	HealthCheck(podFullName string, currentState api.PodState, container api.Container) (Status, error)
 }
 
+// protects checkers
+var checkerLock = sync.Mutex{}
 var checkers = map[string]HealthChecker{}
 
+// Add a health checker to the list of known HealthChecker objects.  Any subsequent call to
+// NewHealthChecker will know about this HealthChecker.
+// panics if 'key' is already present.
 func AddHealthChecker(key string, checker HealthChecker) {
+	checkerLock.Lock()
+	defer checkerLock.Unlock()
+	if _, found := checkers[key]; found {
+		glog.Fatalf("HealthChecker already defined for key %s.", key)
+	}
 	checkers[key] = checker
 }
 
 // NewHealthChecker creates a new HealthChecker which supports multiple types of liveness probes.
 func NewHealthChecker() HealthChecker {
+	checkerLock.Lock()
+	defer checkerLock.Unlock()
+	input := map[string]HealthChecker{}
+	for key, value := range checkers {
+		input[key] = value
+	}
 	return &muxHealthChecker{
-		checkers: checkers,
+		checkers: input,
 	}
 }