This chapter focuses on the thermodynamic cycles in NPPs, primarily analyzing the Rankine cycle used in PWRs and the Brayton cycle applied in HTGRs. It details the fundamental principles, thermodynamic processes, and efficiency calculations for both cycles. The chapter discusses the impact of initial and final steam parameters (pressure and temperature) on cycle thermal efficiency and examines irreversible losses in practical operations (e.g., turbine internal losses, pipeline heat dissipation). Additionally, it explores methods to enhance cycle efficiency, including feedwater regeneration, steam reheat cycles, and optimizations for the Brayton cycle (e.g., regeneration, reheat, and intercooling). Finally, it highlights the potential for cascaded energy utilization through combined cycles (Brayton-Rankine) in NPPs.