The main objective of this work is to analyze the investment in human capital in the development of clusters. The theoretical model uses the concepts of general and specific training proposed by Gary Becker to formalize the tension between the negative effects coming from the opportunistic behavior, empirically identified by the literature, and the positive effects of urbanization and location economies. The results show how cluster sizes, city sizes, and the magnitude of the impact of training on the worker productivity may determine different equilibria.