This is a volume about the process of scientific discovery. Thirteen leading senior scientists, each interested in some aspect of behaviorial development, recount their intellectual journeys over the course of their careers and document their individual struggles to better understand and describe various developmental phenomena. Covering a broad range of topics, including perceptual, motor, social, and cognitive development, the contributors to this volume provide case-studies of how one pursues a long-term, systematic research program and how scientists continually formulate and reformulate their working conceptual frameworks based on their research results. Conceptions of Development provides a unique and personal, behind-the-scenes account of the process of scientific discovery, illustrating that useful and enduring scientific insight derives from the bidirectional interplay between empirical work and theory formulation. This volume will be of interest to a broad audience consisting not only of psychologists and psychobiologists interested in the study of development, but also teachers and students interested in behavioral development and its investigation, and the general reader interested in the process of scientific discovery.