During the lifetime of an internal combustion engine, deposits are formed at various locations. In diesel engines, deposits in the combustion chamber and at the injection nozzles lead to an increase in the emissions, especially the particulate emissions, and the exhaust gas odor. Additionally, durability problems can also arise. Deposits in the combustion chamber of SI engines can increase the octane requirement, deposits at intake valves can reduce engine efficiency and driveability and increase emissions. A detailed theory on the mechanism of deposit formation, considering the physical effects, is presented. This theory contains a deposit transport mechanism, a mechanism of deposit attachment including an induction phase, a deposit growth phase and a deposit removal mechanism. This complex theory is based on fundamental investigations at different locations in and around internal combustion engines. From these investigations, the effects of surface temperature, gas temperature, turbulent gas flow, and gas composition on deposit formation in internal combustion engines will be demonstrated.