Retroviruses package two copies of genomic RNA into one virion. One of the essential steps of replication is reverse transcription, in which the virally encoded enzyme reverse transcriptase (RT) uses the packaged RNA as a template to synthesize viral DNA. Because two copies of RNA are present in one virion, it is possible for RT to switch from one copy of the viral RNA to the other copy during DNA synthesis, thereby generating a recombinant containing some genetic information from each of the RNAs. Recombination occurs at high frequencies during retroviral replication. This frequent recombination has a significant impact on the current human immunodeficiency virus type 1 (HIV-1) epidemic as well as the development of retrovirus-based systems for gene therapy. In this review, the rates, mechanisms, and properties of retroviral recombination are summarized from recent genetic studies. Implications of these studies are also discussed.