Abstract Tooth eruption is a genetically controlled but little understood event. Standard practice has been to divide it into three phases, (1) pre-eruptive. (2) prefunctional and (3) functional. More recently from an orthodontic point of view a six stage division, three stages prefunctional for individual tooth movement and three stages post functional for the whole dentition has been found more useful for clinical purposes. The mechanisms of tooth eruption have been extensively investigated experimentally using the continuously growing and eruption incisors of rodents and lagomorphs both in the impeded and unimpeded state. Four principal theories (1) the alveolar bone growth theory, (2) the root growth and pulp cell proliferation theory, (3) the blood pressure and tissue fluid pressure theory, and (4) the periodontal ligament theory have been advanced to explain the mechanisms of tooth eruption but to date with little success. It has been difficult in these studies to identify the forces which move teeth or the controlling mechanisms of this chronologically precise phenomenon. Cause and effect have not been separated and tooth eruption remains an enigma.