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 thermalhydraulic 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 thermalhydraulics 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. ASME_