The Supercritical Water Cooled Reactor (SCWR) is one of the most promising Generation IV nuclear reactor designs. Operating above the thermodynamic critical point of water this design takes advantage of the unique properties of this single-phase coolant, but also presents numerous challenges for materials. Reactor operating conditions call for a core coolant temperature between 280 and 620°C at a pressure of 25 MPa and neutron damage levels of 15 dpa (thermal reactor design) and 100 dpa (fast reactor design). In the hotter regions of the core, irradiation-induced changes in microstructure (swelling, RIS, hardening, phase stability) and mechanical properties (strength, thermal and irradiation induced creep, fatigue) are major concerns. Throughout the core, corrosion, stress corrosion cracking and the effects of irradiation on them are the most overriding issues. The experience base due to existing light water reactor and supercritical fossil plant operation is combined with the operating requirements for materials in the SCWR to arrive at a set of candidate alloys. This paper will discuss the specific challenges facing these alloys in the SCWR.