During the past 20 years, the pendulum of opinion in the HIV-1 vaccine field has swung between two extremes, initially favoring the induction of antibodies only, and subsequently favoring the induction of cell-mediated immune responses only. At present, the consensus seems to be that induction of both humoral and cellular immunity by an HIV-1 vaccine will be required to achieve maximum protection. One obstacle to the development of an effective HIV-1 vaccine has been the difficulty in inducing broadly reactive, potent antibodies with protective functions. Defining epitopes and designing immunogens that will induce these antibodies is one of the main challenges that currently confronts the HIV-1 vaccine field.