Mental retardation (MR) in children is a heterogeneous group of disorders with varied causes. This article describes well-known causes of MR and epidemiologically established risk factors. Approximately 43–70% of children with severe MR (i.e., intelligence quotient [IQ] of <50) have a known cause of MR, compared with 20–24% of those with mild MR (IQ of 50–70). Investigators will need to continue refining research methods to define homogeneous groups for the further identification of causes of MR in children. Discovery of additional genetic factors and their causal link to MR will continue to diminish the proportion of MR with unknown causes. Areas of MR research that will be particularly challenging are (1) the relationship between socioeconomic factors and other risk factors or causes of MR and (2) how much of the variation in prevalence of MR associated with prenatal or perinatal biologic insults is due to differences in the quality of intervening care and the postnatal environment. MRDD Research Reviews 1998;4:6–13. Published 1998 Wiley-Liss, Inc.