In this review we explore the clinical and scientific status of categorical models of childhood disorder. Three themes are developed. First, the practical origins of standardised category-based diagnostic schemes are examined along with their contemporary philosophical and psychological significance. Next, the impact that these systems have had on the science of child psychopathology is explored. We look at their link to the medical model and the assumption that childhood disorders are categorical, endogenous, and dysfunctional in nature. We argue that these assumptions underpin the dominant paradigm in child psychopathology and so constrain empirical study and theory development. In the final section, the different ways in which researchers have responded to this link and its impact on science are presented. We present the sort of scientific realism associated with Meehl (1995) as the most appropriate basis for a philosophically respectable child psychopathology. Following this approach means unpacking the paradigmatic assumptions, including the assumption of the categorical structure of disorder, into hypotheses that are then put to empirical test. The sorts of data that would allow us to test the categorical hypothesis are identified. We conclude by discussing the results from three recent studies using behaviour genetic analysis of twin data that, in fact, lead us toward a rejection of this hypothesis. The implications for diagnostic and clinical practice of such a rejection are discussed.