Clay minerals are formed by very thin unit layers called “phyllosilicate layers.” They are so thin that around 40% of their constituent atoms are located at the surface. Under certain conditions, surface atoms react with molecules of the surrounding medium, giving rise to the important surface properties of clays. Among all the processes that can take place at a clay surface, ion binding in aqueous media has been a central subject in basic and applied chemistry. The particular case of proton binding to this surface (i.e., its acid-base behavior) has been intensely studied for decades. By accepting or donating protons, clay surfaces acquire electric charge or exchange ions and thereby control the attachment of other ions, molecules, polymers, micro-organisms, and particles. These processes are of primary importance in soil chemistry, environmental chemistry, and geochemistry and have found diverse technological applications. This article deals with the processes that govern the interaction between protons in aqueous media with clay surface groups. By using a rather new interpretation of surface groups reactivity, it explains why different kind of surfaces (basal surfaces and edge surfaces) have different affinity for protons and different charging behaviors and shows how structural charges influence not only the reactivity of basal surfaces, but also that of edge surfaces.