This text considers the development of organisms as a multi-level feedback-linked self-organizing process. Accordingly, the data related to morphogenetic cell activities and to the development of entire organisms are reconsidered and reformulated in terms of symmetry and of a self-organization theory, which is described in non-specialist terms. An important role for biomechanical feedback is emphasized. Two opposing approaches to development are outlined, a "genocentric" one and a "morphocentric" one, and the possibility of their combination is discussed. This book is centered around developmental problems.