Book excerpt: Bacteria are the most ubiquitous of all organisms. Responsible for a number of diseases and for many of the chemical cycles on which life depends, they are genetically adaptable. Vital to this adaptability is the existence of autonomous genetic elements-plasmids-which promote genetic exchange and recombination. The genes carried by any particular plasmid may be found in only a few individuals of any species but can also be shared with other species and thus constitute a horizontal gene pool. This book explains the various contributions that plasmids make to this pool: the replication, stable inheritance and transfer modules, the phenotypic markers they carry, the way they evolve, the ways they contribute to their host population and the approaches that we use to study and classify them. It also looks at what we know about their activity in natural communities and the way that they interact with other mobile elements to promote bacterial evolution.