This book is chiefly intended for those who are using microbicides for the protection of materials. Another purpose is to inform teachers and students working on biodeterioration and to show today's technical standard to those engaged in R&D activities in the microbicide field. When trying to classify, or to subclassify, material-protecting microbicides according to their mode of action, e.g. as membrane-active and electrophilic­ active ingredients, it turned out that a clear assignment was not always possible. For that reason the author has resorted to chemistry's principle of classifying according to groups of substances (e.g. alcohols, aldehydes, ketones, acids, esters, amides, etc.), thus providing the first necessary information about the micro­ bicides' properties. The description of the various groups of substances includes, whenever possible, an outline of the mode and mechanism of action of the active ingredients involved. The effective use of microbicides presupposes knowledge of their character­ istics. That is why the microbicides' chemico-physical properties, their toxicity, ecotoxicity, effectiveness, and effective spectrum are described in greater detail. As mentioned before, the characteristics of microbicides play an important role. They have to be suited to the intended application to avoid detrimental effects on the properties and the quality of the material to be protected; also production processes in which microbicides are used to avoid disturbances by microbial action must not be disturbed by the presence of those microbicides.