Developmental Biology now unites all the biological sciences, and is assuming more and more importance in fields such as medicine and agriculture. For this reason, many more students are required to learn about development as a short module forming part of a larger course. Conscious of the extra pressure this places on students and their teachers, I have tried to write a book that provides a clear summary of the principles of development in a concise and easily manageable format. Development has always been perceived as a difficult subject, and this reflects its origins in the study of diverse organisms using different approaches: simple observation, experimental embryology, genetics and molecular biology. Over the last 10 years, these disparate elements have come together resulting in a new era for developmental biology, which is gene-based and in which the complex cell behaviors that guide development are being dissected at the molecular level. This has shown that fundamentally similar mechanisms guide the development of all organisms, a theme that is emphasized throughout the book. This book focuses largely on the principles of embryonic development, while coverage of post-embryonic development is limited, except in plants. There is also no attempt to describe development in evolutionary terms. This slight reduction in coverage has been necessary to keep the book to an acceptable length and price, but it should still meet most of your course requirements.