Most often when the subject of antimicrobial resistance is discussed, the organizational emphasis is on individual antimicrobial agents or groups of agents. Thus we tend to see discussion of resistance to f3-lactams, tetracyclines, amino glycosides etc. In this book many of the authors were asked to emphasize the mechanism of resistance in their discussion and from that to show how susceptibility to various agents was affected. In part this was done to help emphasize the enormous contribution that the study of antimicrobial resistance has made to our understanding of fundamental physiologic and genetic processes in bacteria. When one looks back over the study of antimicrobial resistance, it is clear that it has been the birthplace of many fundamental advances in molecular biology and of an appreciation of the role of many key functions in the life of a bacterium. In addition, and hopefully to an increasing extent in the future, such study has also contributed to advances in antimicrobial chemotherapy. Through out the book resistance mechanisms have been placed in perspective as to their significance as causes of resistance to key drugs or groups of drugs. Some are of much greater significance than others in terms of the prevalence or the degree of resistance produced. Whatever their numerical significance, however, each of the mechanisms, without question, throws light on fundamental cellular processes and the way in which they interact with antimicrobial agents.