This study investigated the trajectories of preschool and first-grade childrens development of reading skills, as well as the cognitive and social antecedents of that development. One-hundred and ninety-six 5- to 6-year-old children were tested in October and April of their preschool year and again in the first grade. Data included measures of reading ability and its cognitive and social antecedents, which were analyzed using Simplex and Piecewise Growth Curve Modeling. The results showed that during the preschool year individual differences in reading grew larger and that this growth was faster among those who entered preschool with well-developed skills. However, during the first grade individual differences in reading diminished. The results suggest that systematic reading instruction in primary school education is more beneficial for children with less developed literacy skills, whereas children with more developed reading skills gain relatively less from reading instruction in the first grade.