One of the dominant themes of semiconductor device R & D during the 1980's will be the incorporation of heterostructures into most existing kinds of devices, and the emergence of new kinds of devices made possible by heterostructures. In this paper the power of heterostructures as a design tool is illustrated by discussing several ways in which the incorporation of heterostructures can improve the bipolar transistor. The dominant idea is that energy gap variations are a powerful way to control carrier flow; in bipolar structures they permit the control of electrons and holes independently. Several applications of this principle are discussed, going beyond the familiar wide-gap emitter concept, and including several concepts not previously discussed in the literature. The paper closes with a brief discussion of non-bipolar applications and speculative future applications.