Bacteria are the most ubiquitous of all organisms. Responsible for several 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 plasmids in this pool: the replication, the phenotypic markers they carry, the way they evolve, the ways they contribute to their host population and the approaches used to study and classify them.