First published in 1987, this book is about how cells differentiate; that is it describes the way in which cells in animal and plant bodies take on their specialised fates. It has long been recognised that since all such cells retain copies of all genes, the genetic explanation for tissue differences lies in the controlled expression of restricted sets of genes. But how is the choice made and how are such restricted groups of genes activated and regulated? This book discusses these questions and describes both determination and differentiation. The mechanisms that underlie the processes are described for the embryo and in the adult. Determination may occur in adult life during regeneration, wound healing, cancer formation and in the immune and blood systems, and the possible genetic basis for the events is explored. The influence of the cell environment, the cell surface and the pericellular-extracellular matrix as mediators of external signals is discussed.