The powering of nonpowered dams and barriers and the modernization of the aged hydropower plants are emerging practices of hydropower development, due to the minimal impacts on ecosystems and the fact that most of the infrastructure is already in place. The modernization of existing plants also allows to increase safety, resilience to climate changes and to tap new hidden hydropower opportunities. In this chapter, several case studies are discussed on the modernization of existing hydropower plants, showing how their performance (power capacity and annual generation) can be increased by novel strategies and technologies.