A very general definition of acids and basis, following both Bronsted and Lewis theories, is the starting point to introduce the concept of the acid and basic strength of surface sites. The surface of an oxide is described as a bidimensional organization of acid-base pair (AB), whose strength can be measured by probe molecules. It is shown that in order to obtain a reasonable scale of surface acidity and basicity, probe molecules characterized by minimal interaction energy must be used. Among the probe molecules, which can be employed to measure surface acidity, carbon monoxide is the most useful. On the contrary, a probe molecule specific and highly sensitive for basic sites is missing. As case studies, Lewis and Bronsted sites on ZnO and Bronsted acid groups of zeolites are discussed.