This volume brings together authoritative articles, summarizing the current state of knowledge of the genetic organisation of bacterial populations. Contributions include: the evolution of E.coli populations; the roles of gene transfer and recombination; the distribution and evolution of plasmids and antibiotic resistance; population genetics of phase-variable systems; population dynamics and epidemiology of bacterial pathogens; and plasmid transfer in the natural environment. Similarities and differences, within the population genetics of higher organisms, are emphasized throughout, and the impact of bacterial studies on current evolutionary theories highlighted. The consequences of population structure in clinical practice, environmental situations and the commercial exploitation of bacteria are also considered.