Three years have elapsed since the publication of the first edition of this book (in German). The continued interest of our readers and the rapid progress of our knowl­ edge in many fields necessitates a thoroughly revised and somewhat enlarged new edition. Cell differentiation is a prerequisite of life. It is defined as the prooess leading to the qualitatively and quantitatively selective realization of distinct parts of a given genetic material. Cell differentiation comprises five main aspects: (1) signal reception and transformation, (2) selective rearrangements of the genetic material, (3) differen­ tial gene expression, (4) organization of gene expression programs and (5) intercellular coordination of cell differentiation within the developmental programs of multicellular organisms. Despite the bewildering multiplicity of its results, i.e., the differentiated phenotypes of cells, there are apparently fundamental similarities with respect to the molecular mechanisms of the process itself. These mechanisms constitute the central subject of this book.