There is clear evidence that parents can and do influence children. There is equally clear evidence that children’s genetic makeup affects their own behavioral characteristics, and also influences the way they are treated by their parents. Twin and adoption studies provide a sound basis for estimating the strength of genetic effects, although heritability estimates for a given trait vary widely across samples, and no one estimate can be considered definitive. This chapter argues that knowing only the strength of genetic factors, however, is not a sufficient basis for estimating environmental ones and indeed, that attempts to do so can systematically underestimate parenting effects. Children’s genetic predispositions and their parents’ childrearing regimes are seen to be closely interwoven, and the ways in which they function jointly to affect children’s development are explored.