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Rheology is the science of the flow of fluids and deformation of solids. Of special interest to the clay scientist are the flow behavior and stability of clay suspensions, and the time-dependent deformation of clays in a solid or semi-solid state. The physical state of a clay may change with increasing water content; from a solid, to a semi-rigid plastic, then to a gel, and finally to a suspension. In each state, the main factors determining the rheological behavior of the system are related to: (a) the molecular configuration and dynamics of the clay-water interface, and (b) the nature of the particle interactions at this interface. The hydration of the ions and the clay surfaces plays a special role in clay rheology because flow and deformation directly involve molecular movements along the clay-water interface.