A practising clinician is required to use knowledge from many differ­ ent fields. It is unrealistic to expect him to be master of more than a few. In reality, clinicians acquire a smattering of information on most relevant subjects, and learn which texts provide the detailed informa­ tion which is occasionally required on more highly specialized matters. In my professional contacts with clinicians and medical students it has become evident that they often lack the simple frame­ work of microbiological knowledge necessary to guide their actions. This is because standard textbooks and learned treatises alike are con­ cerned with imparting a body of information rather than with present­ ing what the doctor needs to know in order to manage his patients. This volume is an attempt to help clinicians in their everyday practice. To that end I have kept it short and have not dwelt at length even on those topics which especially interest me. No attempt has been made to write a textbook: many of these already exist. A few refer­ ences are given to major reviews and to sources justifying some of the more forthright statements. The subject of medical microbiology is broad and involved. I have therefore seen it as my task to simplify the presentation of the material, being very selective with regard to content and giving my own views on matters of clinical significance.