The purpose of this dissertation was to disentangle the measurement of self-report pubertal timing - the comparative development of an adolescent in relation to peers - in order to determine the longitudinal impact of pubertal timing on substance use across adolescence. Data are from the Context of Adolescent Substance Use study, a school-based longitudinal study of three cohorts, beginning in the 6th to 8th grades (aged 11 to 17, 50% male, 53% White). Study 1 examined the concordance between two self-report measures, stage-normative (based on the PDS) and peer-normative pubertal timing. Kappa statistics were calculated, both as a whole and by demographic subgroup at each age (N=6,425). Most Kappa statistics ranged from poor to modest concordance, indicating that the pubertal timing measures should not be used interchangeably. Study 2 used two longitudinal methods to examine the stability of pubertal timing (N=6,425). When calculating intraclass correlation coefficients (ICC) using one-way ANOVA random effects models, both measures had similar, but poor, stability (stage-normative ICC=.40 and peer-normative ICC=.39). In contrast, latent class analysis (LCA), which determines stability via the underlying response patterns of each measure, showed three stable and distinct response patterns for both measures: always early, always on-time, and always late. Study 3 used latent class growth modeling to test the impact of pubertal timing on current cigarette, alcohol, and marijuana use (N=5,846). Contrasts tested for significant substance use growth model parameter differences between the Study 2 pubertal timing latent classes. For both measures, a higher proportion of early developing adolescents were using substances compared with on-time and, in general, late developers. But using the peer-normative measure, there also was a higher proportion of late developers using cigarettes compared with their on-time peers. The influence on substance use was greatest in early adolescence for both pubertal timing measures and the strength of the relationship was generally stronger using the peer-normative measure. Stage-normative and peer-normative pubertal timing are not synonymous but both are stable throughout adolescence. Early developing adolescents are at greatest risk for substance use and results suggest the social aspects of pubertal development are more influential than the biological aspects.