As a college student, Werner Maas took a course in genetics in 1941 and wondered why so little was said about the biochemical action of genes in controlling the specific function of an organism. Just at that time, biochemists and geneticists began to investigate jointly the basis of gene action, especially in microorganisms. Thus, Maas was able to witness firsthand the spectacular developments that led in the next twenty-five years to a clear picture of the action of genes. The history of these remarkable discoveries is the core of this book. After 1965, building on insights gained from the work with microorganisms, studies of gene action turned to animals and plants and concentrated on processes not present in microorganisms, such as embryonic development, the role of genes in diseases, and the function of the nervous system. Because of the rapidity of technical advances made in handling genes, it has been possible to learn much about these complex processes. The last part of the book deals with these developments, which are ongoing parts of the history of gene action.