The tribological mechanisms pertinent to thin coatings in sliding contacts are described using a systematic approach. The macromechanical, micromechanical and tribochemical changes and material transfer are discussed separately. The tribological process is dominated by the macromechanical mechanisms, which can be analysed systematically on the basis of four main parameters : the coating-to-substrate hardness relationship, the coating thickness, the surface roughness and the debris in the contact. Critical issues relating to the lack of appropriate data and the need for standardized tribological test methods are discussed.