We visualise developmental biology as the study of progressive changes that occur within cells, tissues and organisms themselves during their life span. A good example of a field of developmental biology in which this concept is encapsulated is that of somitogenesis. The somite was identified as the primordial unit underlying the segmented organisation of vertebrates more than two centuries ago. The spectacular discoveries and achievements in molecular biology in the last fifty years have created a gene-based revolution in both the sorts of questions as well as the approaches one can use in developmental biology today. Largely as a result of this, during the 20th and 21st centuries this simple structure, the somite, has been the focus of a deluge of papers addressing multiple aspects of somite formation and patterning both at the cellular and molecular level. One of the main reasons for such interest in the process of somitogenesis stems from the fact that it is such an exquisitely beautiful example of biology working under strict temporal and spatial control in a reiterative manner that is highly conserved across the vertebrate classes. Our intention is that this book will be of interest to different kinds of scientists, including basic researchers, pathologists, anatomists, teachers and students working in the fields of cell and developmental biology. The nine chapters cover a wide array of topics that endeavour to capture the spirit of this dynamic and ever-expanding discipline by integrating both contemporary research with the classical embryological literature that concentrated on descriptions of morphological changes in embryos and the interactions of cells and tissues during development. In so doing they encompass the main aspects of somitogenesis across four vertebrate classes (frog, fish, mouse and chick) and the hope is that this will enable readers to acquire an appreciation of this developmental process in all its facets. Each of the different animal models offers alternative strategic approaches (including experimental embryology, genetics and cell biology) to tackle the same process and as such each offers an invaluable and unique insight into different aspects of somitogenesis. The topics described in these chapters cover the generation of somitic tissue during gastrulation, the molecular mechanisms by which the unsegmented pre-somitic mesoderm becomes segmented into somites, the generation of polarity within somites and the means by which the somite is directed to differentiate into a number of different cell derivatives. There are also two chapters devoted to describing the latest developments on relating spontaneous mouse mutations and mutations leading to abnormal vertebral segmentation in man to the molecular mechanisms already identified as being crucial for somite formation in the lower vertebrates. We would like to heartily thank all of the authors that have contributed their time and effort and whose work has made this book possible. Only they know how difficult it has been to conclude a book that we started more than four years ago, initially with another publisher. One year ago the project was cancelled and then re-started again when the people of Landes accepted to publish it. I am especially grateful for the understanding and patience of Cynthia Conomos and Celeste Carlton who have worked tirelessly to ensure that this project reached completion. We made it!