Acidic catalysis is the acceleration of a transformation by protonation (or by a strongly hydrogen-bonding Brønsted acid) of a reactant, thereby increasing the electrophilicity of that reactant and lowering the barrier to nucleophilic attack or to rearrangement ^[1][2]. In catalysis and oxidation reactions the concept is bounded by an acidic species as the catalytic agent and by a mechanism in which the substrate is activated as an electrophile rather than as a nucleophile. Essential features are a Brønsted acid—molecular or solid—whose proton is transferred or strongly shared in the transition state, and turnover that returns the proton to the catalyst. Typical substrates are glycosyl donors, carbonyl compounds, and alkenes that become oxocarbenium, carboxonium, or carbocation-like intermediates upon protonation ^[1][2]. Sulfonated porous organic polymers illustrate a heterogeneous realization in which strongly acidic sulfonic groups are embedded in a hydrophobic framework that presents the proton to organic substrates ^[3]. The concept is distinguished from acid-base bifunctional catalysis by the absence of a necessary basic site in the catalytic cycle, and from Lewis-acid catalysis by the identity of the activating interaction as proton transfer rather than coordination to a vacant orbital. Conceptual limits exclude stoichiometric protonations that consume the acid and exclude oxidations in which acidity is incidental to a metal-centered redox event.