Systematic research on psychiatric disorders in infants and preschool children (0-5 years old) has lagged considerably behind that for older children, adolescents, and adults. The first step in facilitating such research is developing clearly specified diagnostic criteria that can be reliably applied within standardized assessments across multiple samples. In 2000-2002, a task force of independent investigators developed the Research Diagnostic Criteria-Preschool Age (RDC-PA) for the first time in this age group, with the goal of promoting systematic research on psychiatric disorders. This paper reviews the history of research on psychopathology in early childhood, summarizes the studies on validity, and describes the process behind this effort.