There appears to be no consensus as to what defines an adequate occlusion. Dentists still resort to the generalized concept of static ideal occlusion. However, the concept is questionable in the light of evolutionary theory. During evolution the dentition was subjected to functional wear that is almost non-existent today. Occlusions changed constantly, and cusps did not persist to old age. The recent change in morphology is a possible source of the problems experienced today in function. The low sensory threshold of periodontal mechanoreceptors protects the teeth from trauma. The effect of direct contacts between minimally worn teeth tends to move them not only axially but also in non-axial directions, in analogy with the effect of small hard particles in food. This may disturb the function of the muscles of mastication because adaptation to such contacts was not needed as long as wear eliminated them. Instead of determining the degree of conformity with a static gross morphological norm, actual load distribution on the teeth over the whole range of contact movements appears a valid variable in the assessment of the quality of occlusion. Palabras clave: classification, functional tooth wear, temporomandibular disorders, variation in occlusion