The development of an effective HIV-1 vaccine would be greatly facilitated by knowledge regarding the type and quantity of antibodies that are protective. Since definitive immune correlates are established only after a vaccine has been shown to be effective in humans, animal models are often used to guide vaccine development. Experimental lentivirus infection of non-human primates has shown that neutralizing antibodies can protect against infection. Most specifically, studies of passive antibody transfer in the chimeric SIV/HIV-1 immunodeficiency virus (SHIV) model have provided quantitative data on the level of protective antibody required. While direct extrapolation to humans is difficult, these data likely provide important insights into the protection afforded by antibodies against HIV-1. When used alone, high levels of neutralizing antibodies are required to completely block infection. However, even modest levels of antibody can provide partial protection and affect disease course. Understanding the exact level of protective antibody becomes even more complex in the setting of active immunization and coexisting cellular immunity. Despite this uncertainty, recent findings from lentiviral animal models strongly suggest that neutralizing antibodies will contribute to protection against HIV-1. Based on these data, a major goal of HIV-1 vaccine strategies is the induction of neutralizing antibodies against circulating primary HIV-1 strains.