Abstract: The two FinnTwin studies represent longitudinal twin-family research with ten birth cohorts of adolescent Finnish twins. We review results from this research on genetic and environmental effects and G × E correlations and interactions in patterns of substance use. The FinnTwin studies show that dispositional influences on early substance use and trajectories of later abuse are modulated by sibling and peer interactions and by patterns of parenting. Environmental effects on initiation of smoking and drinking are found to arise from differences between schools and neighborhoods as well as from differences between households. Magnitude and persistence of genetic and environmental effects on substance use differ in urban and rural environments and are influenced by measured community characteristics, including cohesion and surveillance. Multi-stage modeling of trajectories of smoking and drinking reveals a dynamic interplay of genetic and environmental effects from initiation of use, to the development of individual differences in frequency, quantity and density of use, to diagnostic symptoms of abuse. Conjoint developmental trajectories of smoking and drinking appear more heritable than either one modeled alone, providing a heuristic lead for phenotype development in future research with molecular genetic strategies as these 10,000 Finnish twins mature into mid-adulthood.