The quality of a coating depends not only on the physical and chemical properties of its components, but also on the way they are combined and how they interact with each other. Many rheological and colloidal problems are encountered during all phases of manufacture, storage, and application. The role, the causes and the control of pigment/vehicle incompatibility, co-flocculation, reversible and irreversible viscosity drift, macroscopic and microscopic phase separation, and other phenomena are discussed. -- AATA