The acidity of solid surfaces is an important aspect of ion exchange and catalysis. Different techniques have been applied to study the acidity of solid surfaces. Titration methods, the adsorption of basic probe molecules followed volumetrically, gravimetrically, microcalorimetrically, with IR and NMR spectroscopies as well as the desorption of such probe molecules followed by temperature programmed techniques were among the thoroughly applied methods. In this study a brief outline of these studies was made.