The cascaded reservoirs hydropower plant is a complex nonlinear system that involves interacting input and output nonlinear parameters, nonlinear flow rates, and nonlinear dynamical hydraulic heads. This thesis aims to enhance the existing cascaded reservoirs hydropower systems and future ones by new designs of control and optimization systems based on fuzzy logic. In this thesis, a new nonlinear mathematical model of a cascaded reservoirs hydropower plant is developed. The developed model was accurate enough to represent and simulate the plant nonlinear dynamics and to design and test three new fuzzy control systems addressing three major cascaded reservoirs hydropower plant problems. The first one is a fuzzy turbine governor replacing the classical PID controller, the second is a fuzzy controller for three hydraulically coupled turbines under nonlinear and interacting process conditions, and the third is a fuzzy supervisory control and optimization system for geographically-separated systems using the same mass of water in a partially-pumped-storage scheme.