An analysis of the current state of our knowledge of the effectiveness of early intervention for children at risk and for those with established disabilities was provided. A model focused on children's cognitive development was presented in which early intervention is placed firmly within a developmental framework. Both short- and long-term effects, the mechanisms through which early intervention exerts its influence, the relation between those mechanisms and the organization of existing service systems, current limits of programs, and prospects for future research were considered in this model.