Human immunodeficiency virus (HIV), the causative agent of AIDS, belongs to the particularly dangerous and, as a result, the most extensively studied viruses. Until now no effective method protecting against this pathogen has been developed. The major problem is the unusual genetic diversity of HIV, which helps the virus to escape from immunological response and to produce drug-resistant mutants. Most of the collected data suggest that HIV-encoded reverse transcriptase (HIV RT) is the main factor responsible for the continuos generation of new viral variants. There are two primary mechanisms involved in the generation of HIV mutants: high error prone replication and genetic RNA recombination. In this article both processes are discussed in detail.