This text is based on lecture courses given to undergraduates at Liverpool and Manchester Universities, and is intended as an introduction to the biochemistry of antimicrobial action for advanced students in many disciplines. The book is concerned with a discussion of medically important antimicrobial compounds and also a number of agents that, although having no medical uses, have proved themselves as research tools in biochemistry. The aim has been to present the available information in a simple and readable way, emphasizing the established facts rather than more controversial material. Wherever possible gaps in late 1990s knowledge has been indicated.