Vaccine adjuvants are expected to play an important role in enhancing the immunogenicity of existing and new-generation vaccines against infectious diseases. In particular, adjuvants should direct the immune response in the most appropriate manner--furthering, for example, an expanded B-cell response, a cytotoxic T-cell response, or a T-helper 1 or 2 subset response. While some noncytokine adjuvants have exerted potent effects, their modes of action are most likely mediated by cytokines. Several cytokines have already been shown to be efficient adjuvants in animal models and/or in clinical trials. The mechanisms of cytokine function must be better understood and the techniques for the use of cytokines improved if the full potential of these substances as vaccine adjuvants is to be realized. When used to best advantage, such adjuvants enhance the immunity induced by viral, bacterial, and parasitic vaccines and thereby promote efficient protection or even cure.