SINCE THE EARLY DAYS OF MICROBIOLOGY IT HAS BEEN KNOWN THAT, during their life cycles, microorganisms exhibit developmental changes in common with other organisms. In the last decade interest in this aspect of microbiology has developed greatly, and research findings have provided an understanding of the genetic, molecular and biochemical bases of development. An important stimulus in this research has been the realis­ ation that microbial development, in its various forms, provides interest­ ing model systems that have relevance to a much wider understanding of the developmental processes in higher eukaryotes. Many undergraduate and other courses in microbiology reflect these developments. Up to now, the only source material for these courses has been symposia publications, or books of a more specialised nature and at an advanced level. The aim in writing this book, which is based on a series of undergraduate lectures given at the University of Nottingham, was to bring together the relevant aspects of the biology of microorganisms, in particular the bacteria and fungi. The algae and protozoa have been excluded, partly because of the limits of space and partly because they are very different from the bacteria and fungi in most aspects of their biology.