On the Grill: Place frozen bratwurst on the grill. Cook over medium heat (325 degrees F). Grill for 20-25 minutes or until the internal temperature reaches 150 degrees. Turn often as you are grilling.
In the Oven: Preheat oven to 350 degrees F. Place frozen bratwurst in a greased baking dish. Bake for 1 hour and 15 minutes until internal temperature is 150 degrees F.