This publication is the outcome of an IAEA coordinated research project (CRP) on understanding and prediction of thermohydraulic phenomena relevant to supercritical water cooled reactors (SCWRs). The publication illustrates the state of the art of SCWR research and development. It is a key supporting publication for researchers and engineers pursuing the development of SCWRs or equipment/components operating at supercritical pressures. Scientific investigators from participating institutes identified specific research objectives to improve the predictive capability of key technology areas (such as heat transfer and pressure drop for SCWR fuel related geometries, parallel channel stability boundary, natural circulation flow, critical heat flux at near critical pressures, critical flow, and subchannel and plenum mixing). The publication presents the background and objectives and descriptions of the revised Canadian SCWR design concept and a new SCWR design concept being developed at the Nuclear Power Institute of China. It also presents updated information on key areas of technology, such as supercritical heat transfer in simple geometries, stability and critical flow, which have been obtained since the completion of the previous CRP. New experiments and data on supercritical heat transfer in bundles and on critical heat flux, and the application of direct numerical simulation approach for supercritical heat transfer are also detailed.