Neutralizing antibodies represent an important component of immune control in many viral infections. In HIV-1 infection, almost all individuals develop antibodies capable of neutralizing autologous viruses in vitro; however, the role of these antibodies in vivo still remains unclear. Their absence during the acute phase of infection, when the viral levels are brought under control, suggests they play a minor role in immune control and that cellular immune responses are more critical during this time. However, during chronic infection these antibodies may be important in preventing cell-to-cell spread and they still represent our best hope of providing sterilizing immunity (i.e., prevention of infection) by vaccination. Significant advances over the last few years in understanding the structure of the envelope glycoproteins have renewed interest in the role of neutralizing antibodies and the possibility that immunogens capable of stimulating a neutralizing antibody response can be developed.