Solid acid/base catalysis in sub- and supercritical water (Sub&SCW) is a promising method to control organic reactions such as biomass conversions to chemicals and fuels, organic synthetic reactions, and degradation reactions. Synergistically using the unique characteristics of Sub&SCW and the catalytic properties of solid acid/base compounds, high reaction rates and/or high product selectivity can be achieved. This Review provides an overview of the use of solid acid/base catalysts for Sub&SCW reactions. Recent progress in the elucidation of the acid/base catalytic properties of solid catalysts in Sub&SCW and the effect of water properties on the catalysis is introduced. In addition, the application of solid acid/base catalysts in several research fields and the stability of these catalysts during reactions is discussed.