Arrange the clams or mussels around the outer rim of a 10- to 12-inch shallow, round, microwave-proof dish, leaving the center open.
Cover tightly and cook on high (100 percent) for 2 1/2 to 5 minutes, or until all the shells have opened. Rotate the dish once in the middle of cooking time if necessary.
Serve with melted butter, lemon wedges and/or herbs and seasonings, along with the broth from the cooking.