Introduction Why do some people like to run in their leisure time, while other people prefer to watch a video? Are these individual differences in exercise behavior determined by differences in their environment (for example spouse’s exercise behavior, job, school or upbringing) or by differences in genetic predisposition? These are the main questions of this thesis. Furthermore, it is investigated whether there is an association between exercise behavior and psychological well-being and what the cause of this relation is. An important hypothesis is that there is a relationship, but that this association is not causal. For example, differences between people in exercise behavior and psychological well-being might partly be explained by a common underlying set of genes. Firstly, it is tested whether there are genetic influences on well-being, and secondly, it is evaluated whether these genetic influences on well-being overlap with the genetic influences on exercise behavior.