Plasmids drive the evolution of bacteria by conferring novel traits, such as virulence and antimicrobial resistance, in a single step. There is remarkable diversity of plasmids, which can spread within and between bacterial species and genera. In contrast, some human-adapted pathogens, such as Shigella spp. and Neisseria gonorrhoeae, which have long evolutionary histories with a few highly conserved plasmids. The large virulence plasmid was acquired by different species of Shigella as they diverged from Escherichia coli; polymorphisms in this plasmid affect its maintenance, and can promote the development and evaluation of vaccines against this important human pathogen. The gonococcus only carries two resistance plasmids which are rarely found in other species; the ongoing evolution of these plasmids has a significant impact on public health.