Evidence on the interrelation of intelligence development and the development of domain-specific academic achievement is still inconclusive. We investigated the longitudinal relation between these 2 constructs in the domains mathematics and reading. Data from 6 large adolescent student samples (Ntotal = 24,828) from 4 longitudinal studies were analyzed using an integrated approach. Continuous time models corroborate the assumption that intelligence and academic achievement are reciprocally related over the course of 9 months, a time period that approximates the length of a school year. Reciprocal relations were observed regardless of the achievement indicator employed (standardized test score or report card grade) and the academic domain. Multigroup analyses demonstrated that the strengths of associations between intelligence and the indicators of academic achievement was robust across sexes. Our rigorous tests of the interrelation between intelligence and academic achievement underscore the importance of adolescents' learning opportunities not only for achievement in academic domains, but for intelligence development more generally.