Shellfish (crustaceans and mollusks) have long been known as a common cause of allergic reactions to food. Like other food allergies, the allergic reactions to shellfish involve IgE-mediated Type I hypersensitivity. Biochemical and molecular studies have documented the major shrimp allergen is the muscle protein tropomyosin. Subsequent molecular cloning studies on lobsters and crabs have characterized this protein as the common allergen in crustaceans. There has also been strong immunological evidence that tropomyosin is a cross-reactive allergen among crustaceans and mollusks. This is further confirmed by recent studies on the identification of allergens in squid and abalone. The advances in the characterization of shellfish allergens will not only enhance our understanding on the physiological basis of shellfish allergy but also lay the groundwork for the development of diagnostic and therapeutic design in food allergies.