This study assessed genetic and environmental contributions to temperament during adolescence within the Nonshared Environment and Adolescent Development project (NEAD; D. Reiss, J. M. Neiderhiser, E. M. Hetherington, & R. Plomin, 2000). NEAD is a national study that includes twins and other sibling types who vary in regard to genetic relatedness. Seven hundred twenty sibling pairs (aged 12.1-13.5 years) participated at Time 1, and 395 sibling pairs (aged 14.7-16.2 years) participated again at Time 2. At both Times, mothers and fathers rated their children's temperament (emotionality, activity, sociability, and shyness). At Times 1 and 2, genetic and nonshared environmental factors accounted for variance in temperament, whereas shared environmental contributions were negligible. However, at Time 1, genetic contributions were inflated, and shared environmental contributions were masked if sibling contrast effects were not taken into account. At Time 2, sibling interaction effects had little impact on estimates of genetic and environmental contributions to temperament. Last, temperament stability was primarily explained by genetic factors, whereas both genetic and nonshared environmental factors accounted for change.