To review and discuss the aetiology, prevalence and classification of this dental anomaly together with the morphology of an invagination and the most appropriate nomenclature. This review considers the different possible nomenclatures and concludes that dens invaginatus is the most appropriate description. The paper highlights the different reported prevalence figures and concludes that the problem is probably one of the most common of the dental developmental abnormalities with maxillary lateral incisors most commonly affected. The paper suggests that the classification system described by Oehlers (1957a) is probably the most clinically relevant and that the morphological features associated with this problem may increase the risk of pulpal pathology developing and complicate any possible endodontic treatment. * The aetiology of dens invaginatus is still unknown, although there is some evidence that it may be genetic in origin. * The problem is probably more prevalent than most clinicians are aware of and this is because of the diagnostic difficulties associated with the anomaly. * The nature of the problem may increase the risk of pulp disease and complicate any root canal treatment.