The authors examined the developmental course of self-regulation in a cohort of children from the National Longitudinal Survey of Youth. The longitudinal sample included 646 children (48% girls; 52% boys; 36.2% Black, 23.4% Hispanic, 40.4% White) who were 4 to 5 years old in 1986 and who were followed up at ages 8 to 9 and ages 12 to 13. Levels of self-regulation (assessed with 12 maternal-report items that measured regulation of affect, behavior, attention) increased from early childhood (when sample children were 4 or 5 years old) to middle childhood (ages 8 or 9), but not from middle childhood to early adolescence (ages 12 or 13). Girls exhibited significantly higher levels of self-regulation than did boys at all 3 time points. Individual differences in self-regulation were fairly stable across the 8-year span (rs =0.47 to 0.50). Comparisons of 1-, 2-, and 3-factor models suggested that the different aspects of self-regulation are highly interrelated, and support adoption of a single-factor model for both genders. The authors discuss implications of these findings for theory and intervention. (Contains 3 tables and 2 figures.)