Few studies have investigated how preschool and primary school interact to influence children's cognitive development. The present investigation explores German children's numeracy skills between age 3 (1st year of preschool) and age 7 (1st year of primary school). We first identified the influence of preschool experience on development while controlling for child factors, family background, and the quality of the home learning environment (HLE). We then considered how the instructional quality of primary schools influences numeracy. We finally analysed how preschool and primary school interact. We sampled 547 children who attended 97 German preschools. Latent growth curve analyses identified child and family factors related to age 3 numeracy and development to age 7: gender, migration background, socioeconomic status (SES), mother education, HLE. The effects of preschool on numeracy development persist until age 7 with notable effects from process quality. Strengthened efforts are needed to ensure high quality preschool education in Germany. (Contains 4 tables, 2 figures and 4 notes.)