Since its discovery in 1983, a lot of knowledge about the human immunodeficiency virus (HIV) has been accumulated. Our paper gives a brief survey on what is known at present about the structure, molecular biology, and the cell tropism of this virus. We discuss its relationship to other lentiviruses as well as its possible origin; in addition, we refer to the immune response to HIV and its interactions with the infected host. We also briefly summarize the difficulties encountered in the attempts to produce a vaccine against HIV and highlight some promising approaches in the development of such a vaccine.