AbstractThe synthesis and integration of DNA into the genome of its host cell is a normal step in the replication of the retroviruses. Previous studies have provided details concerning the structure of viral DNA and viral and host integration sites. More recent genetic and biochemical results have expanded our understanding considerably: it should soon be possible to describe the exact viral DNA sequence recognized during the integration reaction for several viruses. In addition, at least one of the viral proteins and enzymatic activities required in the reaction has been identified. Analysis of this apparently efficient and highly specific site‐directed recombination event in eukaryotic cells promises to provide insights of both fundamental and practical interest.