The mechanisms that control region-specific and axial patterning of cell differentiation during embryogenesis have fascinated experimental biologists for over 100 years. This book describes recent advances in our knowledge of the cellular and molecular mechanisms of pattern formation. The book describes research on the two main conceptual frameworks within which patterning is currently analysed: positional information and induction. The developmental time frame covered extends from the oocyte to developing organ systems, and a wide range of research organisms, from hydrozoans to birds, is represented.