Abstract: The Twin and Offspring Study in Sweden (TOSS) is a study of 909 pairs of twins who are parents and includes the twins, their adolescent child and their spouse or partner. The TOSS was designed to address three key questions: (1) what are the genetic and environmental influences on parenting for a sample of twins who are parents; (2) what are the genetic and environmental influences on adult family relationships; and (3) how do family relationships impact the adjustment of adults. Because of the inclusion of the children of twins as well as the twins themselves, the TOSS is able to begin to specify the types of genotype-environment correlation, especially when paired with other studies examining children who are twins. Findings from the TOSS have helped to clarify the types of genetoype environment correlation involved in parenting and other family relationships, have helped to specify how individual characteristics of parents influence their parenting, and how such individual characteristics can explain genetic influences on family relationships. Future directions for analyses using the TOSS include additional specification of genotype-environment correlation, examination of genotype × environment interaction and the analysis of specific genes for associations with the environmental constructs measured in TOSS and specific gene × specific environment interactions.