This book is based on Hans Zahner's Biologie der Antibiotica, published in 1965. There is a vast literature on antibiotics, covering chemical, phar macological, and clinical aspects. We have made no attempt to cover this literature comprehensively. Our effort is directed toward discuss ing antibiotics as biological agents. They are substances produced by living cells, yet they are able to inhibit the growth of living cells - in many cases even the cells that produce them. We have taken this apparent biological paradox as our point of departure and have tried to look in this light at the production of antibiotics and at their mode of action. In a sense antibiotics are comparable to mutations. They are useful as tools in the study of metabolism by blocking specific reactions. At the same time their mode of origin and their effects on the organisms that produce them are interesting problems in their own right. We have tried to incorporate both aspects into our consider ations. This little book, designed for biology students and medical stu dents, provides them with a framework into which to fit more specialized and detailed information on antibiotics.