Although it is generally believed that early intervention programmes are an effective means to stimulate children's cognitive development, many questions remain concerning programme design and delivery. This article reviews 19 studies into the effectiveness of early intervention programmes published from 1985 onward. The database comprised 85 different outcomes or effect sizes (71 in the cognitive domain, 14 in the socioemotional domain). The overall effect size estimate was d = 0.32 (SE = 0.05) in the cognitive domain, and d = 0.05 (SE = 0.02) in the socioemotional domain. Effect sizes were found to depend on delivery mode. Centre-based interventions and interventions following the combined home- and centre-based delivery mode produced greater effect sizes than did home-based programmes in the cognitive domain, but not in the socioemotional domain. The programme inclusion of coaching of parenting skills was also positively related to outcomes in the cognitive domain. Several other programme characteristics, including age of onset, programme length and intensity, continuation after kindergarten, and the inclusion of social or economic support, appeared not to be uniquely related to outcomes. (Contains 5 tables.)