Plasmids are closed, circular pieces of DNA which are able to self replicate and are carried by many bacteria. They provide unique functions for the bacteria, e. g. by allowing them to sexually replicate and to pass on genetic material between each other. Plasmids are also responsible for the genetic factors that give resistance to antibiotics, and provide the enzymes needed to break down poorly metabolised food resources. The author has provided an up to date treatment of the structure, function and application of plasmids suitable for undergraduates and medical students. An original teaching perspective is employed in that plasmids are examined as living organisms with either a symbiotic or parasitic mode of survival. This gives a very important frame-work for understanding the structure and function of plasmids in an evolutionary context.