This study investigated the development of academic self-concept and language achievement from Grade 7 to Grade 12 by repeated assessment of 2826 Flemish adolescents in 50 secondary schools. Latent growth curve modeling showed that both girls and boys experience a declining academic self-concept during the period of secondary education and that girls declined at a faster rate. Furthermore, girls were shown to have an increase in Dutch language achievement over time, whereas boys showed a decrease in middle years, followed by an increase from Grade 9 on. The multivariate multilevel growth curve model suggested that the evolution of academic self-concept was not related to the evolution in achievement, neither at the individual level, nor at the school level. There is, however, a positive relation between students' academic self-concept and their achievement, the magnitude of which decreased throughout secondary school. At the school level, the correlation is small, but also positive, except for the girls from Grade 10 on. The results are discussed in relation to the reciprocal effects model and the developmental perspective regarding the self-concept/achievement relation.