A large number of empirical studies have documented that schooling and training display a strong complementary relationship. However, the mechanisms driving the training gap between individuals with high and low levels of schooling are not yet well understood. We contribute to the literature by analyzing whether the observed complementarity is causal or due to unobservable time-invariant factors. Furthermore, we consider several regional factors that potentially determine training participation, e.g. firm competition, the availability of training subsidies as well as the regional supply of training by providers. We pay specific attention to heterogeneous effects by level of schooling. Finally, we will estimate the returns to training and test for heterogeneity in returns by level of schooling. In addition to monetary returns, e.g. by means of wage increases, we will consider several non-monetary returns, e.g. job security or job satisfaction.