In every branch of science, it is necessary to concentrate on a particular group of problems at once, which means that areas of inquiry must be delimited. Yet, any delimitation of an area inevitably introduces difficulties and dangers. First, any boundary lines that are used to separate one set of natural phenomena from others must, to some extent, be artificial and, thus, potentially misleading. This is particularly clear with reference to psychological development. On the one hand, all psychological processes have developmental aspects. All of them can and, if they are to be fully understood, should be studied developmentally. On the other hand, the developmental psychologist finds himself willy-nilly making encroachments on the preserves of general psychologists, abnormal psychologists, social psychologists, and comparative psychologists. Second, even when the distinctions between areas of inquiry are relatively clear-cut and generally agreed on, there is always a need for efforts at integration. Any two phenomena in the universe must be alike in some respects and different in others. Science has not succeeded in its tasks until it has related any phenomenon that is under consideration to other phenomena, until it has pointed out similarities and differences. The greatest triumphs in the history of science have come from finding links between things that seemed to belong to utterly disconnected domains. It is especially important to relate complex phenomena to simple ones, both because the principles and processes that are common to the simple and the complex are likely to emerge most clearly when the simple is studied and because the nature and sources of the complexities that distinguish the complex from the simple become manifest most readily when simple and complex are compared. With admirable complementarity, Dr. Kessen and Dr. Spiker have addressed themselves primarily to the delimitation of developmental psychology within psychology and to the delimitation of cognitive development within psychological development, respectively. I find myself in strong agreement