Most industrial applications of zeolites as catalysts rely on their Bronsted acidity properties. However, neither quantifying acid strength in solids nor correlating acidity with catalytic activity is straightforward. The ease and speed at which the proton transfer process occurs in zeolites depends on different factors, including the concentration of active sites and local geometry around them, the ability of base molecules to diffuse through channels and cavities close to the acid sites, and the stabilization of ionic transition states and intermediates by electrostatic interactions with the zeolite framework. All these aspects are analyzed in this review based on experimental characterization data (FTIR spectroscopy of hydroxyl groups and adsorbed probe molecules, in situ MAS-NMR of reactants and intermediates, TPD and microcalorimetry of adsorbed probe molecules) and computational studies.