Genetic and environmental sources of continuity and change in aggression were studied in a sample of 1,041 twin pairs (364 monozygotic; 348 same-sex dizygotic; and 329 opposite-sex dizygotic) as part of an ongoing, population-based Finnish twin-family study. At ages 12 and 14, the twins' aggression was assessed by their classroom teachers, using a rating form of the Multidimensional Peer Nomination Inventory. Genetic and environmental sources of continuity and change were studied by fitting a longitudinal bivariate Cholesky decomposition model. Longitudinal model-fitting results indicated that both genetic and environmental factors influenced continuity in aggression during this 2-year period, but the age-to-age correlation of these factors differed by sex. Continuity in boys' aggression was mediated by genes and common environmental factors; in girls, in contrast, continuity was due primarily to common environmental, and to a lesser degree, unique environmental factors. Genes and unique environments contributed to change in aggression in both sexes. Aggr. Behav. 31:1–13, 2006. © 2006 Wiley-Liss, Inc.