Over the past several years, the progress made in understanding the cellular immune response to HIV is likely greater compared to any other time since the beginning of the worldwide epidemic. This progress has largely been made possible by technical advances that have permitted a much more quantitative and highly detailed study of virus-specific cellular immunity in humans than was previously available. However, despite intensive study of the HIV-specific cellular immune response, we do not fully understand the nature of immunologic control in some rare cases and lack of control in most of untreated patients. It has become increasingly clear that HIV replication is poorly controlled in most untreated patients, despite a high-frequency HIV-specific cellular immune response. Therefore, attention has turned to qualitative features of the immune response that may dictate restriction of viral replication. Because most vaccines in preclinical or clinical testing rely on cellular immune responses that may alter disease progression but are unlikely to prevent infection, understanding these qualitative features is of particular importance. Further study could yield critical information for inducing effective immunity in vaccinees, preventing the loss of control of viral replication on the infection of vaccinees, or inducing immunologic control in infected humans.