It is now well established that a large majority of small, multicopy plasmids of Gram-positive bacteria use the rolling-circle (RC) mechanism for their replication. Furthermore, the host range of RC plasmids now includes Gram-negative organisms as well as archaea. RC plasmids can be broadly classified into at least five families, individual members of which are spread among widely different bacteria. There is significant homology in the basic replicons of plasmids belonging to a particular family, and there is compelling evidence that such plasmids have evolved from common ancestors. Major advances have recently been made in our understanding of plasmid RC replication, including the characterization of the biochemical activities of the plasmid initiator proteins and their interaction with the double-strand origin, the domain structure of the initiator proteins and the molecular basis for the function of single-strand origins in plasmid lagging strand synthesis. Over the past several years, there has been a 'renaissance' in studies on RC replication as a result of the discovery that many plasmids replicate by this mechanism, and studies in the next few years are likely to reveal new and novel mechanisms used by RC plasmids for their regulated replication.