Many HIV-1 isolates contain interwoven genomic sections derived from multiple parental strains. Such chimeric genomes arise via genetic recombination. This review summarizes experimental approaches for addressing the frequency of HIV-1 genetic recombination during single cycles of viral replication in vitro, and describes factors--such as variation in extents of sequence homology and the metabolic state of the infected cell--that modulate recombination. Findings from such studies suggest that recombinogenic template switching is an even more common occurrence during HIV-1 DNA synthesis than is the introduction of base substitution errors. This implies that recombination is an inherent property of retroviral DNA synthesis, and that the vast majority of HIV-1 DNAs are biochemical recombinants.