Basophils, the least abundant granulocytes, have poorly understood functions. They have been linked to the development of T helper type 2 immunity during parasite infection and allergic inflammation. Emerging evidence has not only shown the critical involvement of basophils in the development of T helper type 2 immunity but also provided useful animal models with which basophil functions can be further examined. However, distinctions must be made between what basophils 'can do' after in vitro manipulation and what they 'actually do' during in vivo immune responses; these may be very different. In this review, the functions of basophils determined on the basis of analysis of in vitro and in vivo systems and their potential involvement in clinical settings are discussed.