Objective: To investigate the degree to which genetic and environmental influences affect individual differences in sedentary behavior throughout adolescence. Design: Cross-sectional twin-family design. Setting: Data on self-reported sedentary behavior from Dutch twins and their nontwin siblings. Participants: The total sample consisted of 5074 adolescent twins (aged 13-19 years) and 937 siblings (aged 12-20 years) from 2777 families. Main Outcome Measures: Screen-viewing sedentary behavior was assessed with survey items about weekly frequency of television viewing, playing electronic games, and computer/Internet use. Based on these items, an overall score for screen-viewing sedentary behavior was computed. Results: The genetic architecture of screen-viewing sedentary behavior differed by age. Variation in sedentary behavior among 12-year-olds was accounted for by genetic (boys: 35%; girls: 19%), shared environmental (boys: 29%; girls: 48%), and nonshared environmental (boys: 36%; girls: 34%) factors. Variation in sedentary behavior among 20-year-olds was accounted for by genetic (boys: 48%; girls: 34%) and nonshared environmental (boys: 52%; girls: 66%) factors. Conclusion: The shift from shared environmental factors in the etiology of sedentary behavior among younger adolescents to genetic and nonshared environmental factors among older adolescents requires age-specific tailoring of intervention programs.