This monograph summarizes the major developments on nuclear reactor thermal-hydraulics over the last fifty years, primarily for the water-cooled reactors, and provides a direction for the future thermal-hydraulic developments for water-cooled, including small modular reactors or SMR, and Generation IV reactors. This includes discussion on the steady-state reactor thermal hydraulics including subchannel analysis, evolution of emergency core cooling systems (ECCS) from active to fully passive systems to remove the decay heat, and development and consolidation of the best-estimate safety analysis methodology. With substantial increase in computing power, the computational fluid dynamics (CFD) tools for single-phase and multi-phase flows are being used more these days to address some of the important reactor thermal-hydraulics phenomena which could not be analyzed earlier using the traditional one-dimensional or coarse three-dimensional analysis tools. Development of multi-physics methodology encompassing neutronics, thermal-hydraulics, thermal-mechanical and coolant chemistry has also started.