Our understanding of the molecular mechanisms involved in mammalian brain development remains limited. However, the last few years have wit­ nessed a quantum leap in our knowledge, due to technological improve­ ments, particularly in molecular genetics. Despite this progress, the available body of data remains mostly phenomenological and reveals very little about the grammar that organizes the molecular dictionary to articulate a pheno­ type. Nevertheless, the recent progress in genetics will allow us to contem­ plate, for the first time, the integration of observation into a coherent view of brain development. Clearly, this may be a major challenge for the next century, and arguably is the most important task of contemporary developmental biology. The purpose of the present book is to provide an overview that syn­ thesizes up-to-date information on selected aspects of mouse brain development. Given the format, it was not possible to cover all aspects of brain development, and many important subjects are missing. The selected themes are, to a certain extent, subjective and reflect the interests of the contributing authors. Examples of major themes that are not covered are peripheral nervous system development, including myelination, the development of the hippocampus and several other CNS structures, as well as the developmental function of some important morphoregulatory molecules.