The present understanding of critical heat flux (CHF) in subcooled flow boiling with water is reviewed and fusion reactor component high-heat flux (HHF) requirements are outlined. This survey (Parts I and II), which contains a representative coverage of the literature over the last 30 years, is concerned only with CHF in the subcooled flow boiling regime. Although not exhaustive, CHF data base parameter ranges are also given as an aid for fusion component designers in locating the appropriate data for an application. Because of the relatively HHF levels and long pulse durations in the next generation reactors, fusion components must be actively cooled. All fusion components are heated nonuniformly over their surface and their surface area ranges from 0.1 to 1000 m2. Although most components are subjected to fluxes from ∼0.005 kW/cm2 (first wall) to near 1 kW/cm2 (limiters and divertors), some components are subjected to fluxes from 2 kW/cm2 (first wall in compact reactors) to 8 kW/cm2 (beam dumps). Subcooled flow boiling has the greatest potential of accommodating the steady-state HHF levels encountered by fusion reactor components. Although the available heat flux data base brackets those for most fusion components, the existing data are sparse or nonexistent for the length-to-diameter ratios (e.g., >200 for limiters and >50 for beam dumps) necessary for future HHF fusion components. There are more than 20 parameters that influence subcooled flow boiling CHF and many other tested techniques that enhance heat transfer by a factor of >2. The engineering implementation and design of fusion components cannot be optimized until the physical relationships between the maximum CHF and both the flow parameters and thermophysical properties have been determined. This can be accomplished only if improvements are made in the understanding of the fundamental mechanisms controlling the heat transfer and CHF in the subcooled flow boiling regime.