The electrical double layer at a solid-liquid interface plays a prominent role in the interpretation of many colloidal phenomena. Adsorption of foreign ions at colloidal particle surfaces is influenced by the surface charge density ([sgrave]0) and the double layer potential (0). The stability of hydrophobic colloidal solutions is largely controlled by the relative strength of repulsive and attractive (van der Waals) forces which come into play when the double layers of two approaching particles overlap.