It has been known for many years that not all individuals who are repeatedly exposed to HIV-1 show evidence of viral replication, seroconvert, and eventually develop disease. Quite apart from those who seroconvert but progress slowly to AIDS (ie, slow progressors, longterm nonprogressors, elite controllers), these rare, exposed seronegatives either resist infection or harbor extremely low levels of virus that may be detected only using ultrasensitive methods (occult infection). The correlates of protection that confer this unique status to a tiny minority of HIV-exposed individuals remain a subject of intense interest, investigation, and controversy, as no single genetic or immunologic parameter has yet been able to fully explain this phenomenon. However, there is general consensus that studying these individuals may provide invaluable information to aid in the design of vaccines and therapeutic approaches. This review describes the major findings on this important topic, with a focus on immunologic studies.