The envelope of human immunodeficiency virus (HIV) is an essential building block of the virus and it plays a major role in its life-cycle, particularly during the early stages of infection. It very likely determines, at least in part, the host range and tissue specificity of HIV, participates in pathogenic processes mediated by the virus and can itself be immunosuppressive. Because of its strategic location on the outer surface of the virion and the infected cell, it also represents an optimal (although not the only) target for immune attack and thus a prime candidate for development of vaccine and therapeutic strategies. Efforts to better understand its structural, functional and antigenic properties will thus be well worthwhile. Some of its principal features are reviewed herein and its role in vaccine strategies is discussed.