A review of in vivo genetic systems covers the key features of transduction and conjugation but emphasises the intramolecular and intermolecular DNA interactions that are often associated with these processes. As well as the transfer of many lactose plasmids, conjugal transfer of nisin genes and the use of conjugation to construct bacteriophage-resistant dairy starter cultures are discussed. The discovery and characterization of insertion sequences in Lactobacillus and Lactococcus and the exploitation of heterologous conjugation and transposition systems in the lactic acid bacteria are described.