L-form bacteria include both stable and unstable cell wall-less variants of native walled bacteria. These organisms have been implicated as etiologic agents in both plant and animal diseases. L-form bacteria may also be used to protect plants or animals against disease-causing organisms, or to act as a host bacterium for the production and secretion to the growth medium of recombinant proteins. This article summarizes the fundamental biology of these unusual bacteria and discusses their potential uses.