Studies of conceptual development increasingly address children's understanding of physical, biological, and psychological phenomena. These are three major domains of understanding in that they encompass much of the external world with which adults and children interact--the worlds of people, plants, and animals, and physical objects and occurrences. There is considerable current debate concerning what children know about these domains as well as how and when they develop that knowledge. Interestingly, amid all the studies and debate, seldom does research focus on direct comparisons across these types of thinking; this volume of New Directions for Child Development does so. The contributors to this sourcebook compare the development of reasoning in these three domains in normal children from several different countries and cultures and also in retarded, deaf, and autistic children. This is the 75th issue of the quarterly journal New Directions for Child Development.