Skeletal muscle development is perhaps one of the best understood processes at the molecular, cellular and organismal level due in large part to the fact that primary myogenic cells (myoblasts) will grow and subsequently differentiate into myotubes in culture. With the advent of reverse mouse genetics, many of the observations gained through the study of myogenic cells in vitro have been directly tested in vivo . What has emerged is a complex but cohesive story of how myogenic cells are initially specified in the vertebrate embryo and how muscle fibers ultimately achieve their respective identities (i.e. fast versus slow) to perform their function. This collection of chapters is focused on these developments. The book discusses old and new directions for the skeletal muscle field and points out directions where the field may eventually progress.