It is well documented that bacteria can readily exchange genetic information under artificial conditions typically used in most laboratory studies as well as to some extent in nature. The three mechanisms by which such genetic exchange can occur are transformation, transduction and conjugation. Transformation is the uptake of free DNA into a cell from the surrounding environment, while bacterial viruses mediate the exchange of genetic material during transduction and conjugation involves the direct transfer of DNA during cell-to-cell contact. In most cases, plasmids mediate the transfer of DNA during conjugation events, although chromosomal transfer can also occur. This review will focus mainly on plasmids and the role of conjugation in marine sediment microbial communities. Plasmids, although often dispensible, provide a unique plasticity to an individual host cell or to an entire microbial community ‘genome’. Specifically, plasmid-encoded traits mobilized throughout microbial communities can provide a means of rapid adaptation to changing environmental conditions. Examples of such adaptation can be seen in the increased frequencies of catabolic plasmids and antibiotic and heavy metal resistance plasmids within microbial populations upon exposure to selective pressures. Presently, the view of plasmid diversity and horizontal transfer dynamics is predominantly based on broad- and narrow-host-range plasmids isolated from bacteria of clinical and animal origins. While the exchange of plasmids is most likely an important mechanism by which bacterial populations in clinical environments can evolve and adapt, there remains a general lack of information regarding the role of plasmid-mediated transfer in marine ecosystems and how indigenous plasmids impact the microbial community structure and function. The combined application of molecular biology and microbial ecology techniques is providing new approaches to address the ecological role of plasmids in marine environments.