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Solid acid catalysis is the acceleration of a chemical transformation by Brønsted or Lewis acidic sites that are localized on an insoluble solid, so that the catalyst occupies a separate phase from the reactants ^[1][2]. In synthesis and catalytic reactions the concept is bounded by a heterogeneous acid—zeotype, heteropolyacid, sulfonated polymer, metal oxide, or related solid—and by protonation or coordinative activation of the substrate at that solid surface. Essential features are an accessible acidic site whose strength and local environment determine the elementary proton-transfer or hydride-transfer step, a porous or high-area architecture that presents those sites, and the absence of a dissolved molecular acid as the catalytic species ^[2][3]. Silver-exchanged zeolites and heteropolyacids illustrate how a cation associated with the solid acid can modify the protonic reactivity without converting the catalyst into a soluble acid ^[3]. The same solid-acid sites mediate carbonyl–nitrogen condensations and biomass-derived dehydrations when the substrate can adsorb at the protonic center ^[1][4]. The concept is distinguished from homogeneous Brønsted-acid catalysis by the phase-separated acid and by the additional constraints of pore size, site isolation, and adsorption equilibria. Conceptual limits exclude solid bases and exclude solids that function only as supports for a molecular acid that leaches into solution.