Effective preventive and therapeutic intervention in individuals exposed to or infected with human immunodeficiency virus (HIV) depends, in part, on a clear understanding of the interactions between the virus and those elements of the host immune response which control viral replication. Recent advances have provided compelling evidence that cytotoxic T lymphocytes (CTLs) constitute an essential component of protective antiretroviral immunity. Here, we review briefly the significance of this work in the context of previous studies, and outline the mechanisms through which HIV evades CTL activity.