Genetic recombination is common to all forms of life and involves the exchange of DNA sequences between two chromosomes or DNA molecules. Such exchanges contribute to the generation of genetic diversity and the repair of damaged DNA. There are two major classes of recombination, site-specific recombination and general or homologous recombination. In homologous recombination the joining of the DNA duplexes exhibits a similar degree of precision or fidelity but, generally speaking, does not take place at specific sites. Since exchange can occur anywhere along the length of two homologous chromosomes, it follows that the proteins that catalyze homologous recombination are not sequence- or site-specific binding proteins. This review focuses on genetic and biochemical analyses of homologous recombination proteins that carry out conjugational recombination in E. coli and meiotic recombination in eukaryotes.