Lower socioeconomic status (SES) is widely accepted to have deleterious effects on the well-being and development of children and adolescents. However, rigorous meta-analytic methods have not been applied to determine the degree to which SES supports or limits children's and adolescents behavioural, cognitive and language development. While SES is largely determined by combinations of variables such as parental education level, marital status, and income, SES may also interact with other variables mediating or moderating the influence of SES on children's behavior and cognitive development. Thus, the objective of this study was to conduct a meta-analysis of research on the relationship between composite measures of SES and developmental outcomes for children and adolescents between the ages of birth to 19 years of age. The results revealed very small to small, but significant effects of SES on aspects of the three outcome variables of literacy and language, aggression, and internalizing behaviours including depression. Many other factors come in to play that may better explain the small, but significant relationship between SES and development. Given the small observed associations, policy makers and programmers may focus interventions on family and community factors that contribute to child and adolescent developmental outcomes across the socioeconomic spectrum. (Contains 1 figure and 3 tables.)