More than 25 years ago the thesis ‘Functional occlusion and mesial drift, an experimental study of tooth migration in monkeys’ was presented. A literature review was carried out to assess whether the results have withstood the test of time. It was confirmed that occlusion plays an important role in mesial drift, whilst no direct evidence for transseptal fibre contraction as a causal factor has been put forward. Mesial drift is not merely a compensatory mechanism for loss of tooth susbstance, but may cause crowding, depending on axial inclination and occlusal morphology. This phenomenon has implications for orthodontic and restorative treatments.