The Effective Pre-school Provision in Northern Ireland (EPPNI) project is a longitudinal study of child development from 3 to 11 years. It is one of the first large-scale UK projects to investigate the effects of different kinds of preschool provision, and to relate experience in preschool to child development. In EPPNI, 683 children were randomly selected from 80 preschools, and 151 children were recruited without preschool experience. Progress was then followed from age 3 to age 11. Preschool experience was related to age 11 performance in English and mathematics. High-quality preschools show consistent effects that are reflected not only in improved attainment in Key Stage 2 English and mathematics but also in improved progress in mathematics over primary school. Children who attended high-quality preschools were 2.4 times more likely in English, and 3.4 times more likely in mathematics, to attain Level 5 than children without preschool experience. (Contains 4 tables.)