This study examined genetic and environmental effects on individual variation in pubertal timing using two national samples of siblings from the Nonshared Environment of Adolescent Development (NEAD) and the National Longitudinal Study of Adolescent Health (Add Health). In each sample, female and male siblings with different degrees of genetic relatedness, i.e., monozygotic twins, dizygotic twins, full siblings, half siblings, and unrelated siblings in blended families, were assessed. Timing of pubertal development was measured by age-adjusted self-report measures of the Pubertal Development Scale in NEAD and a four-item scale of pubertal development in the Add Health. The results indicated that both genetic and environmental influences play an important role in determining the relative timing of pubertal development for both boys and girls. (Contains 1 figure and 6 tables.)