The objective of this article is to review the process of permanent tooth eruption and primary tooth resorption in the context of the major theories of tooth eruption. This review will also focus on diagnostic criteria including radiographic, clinical, and genetic analysis to help distinguish between eruption anomalies such as isolated ankylosis of primary or permanent teeth versus eruption failure. Our studies of eruption failure have revealed that a much greater number of cases than previously suspected account for a hereditary form of eruption failure versus an isolated ankylosis. The importance of distinguishing eruption failure from isolated ankylosis (in primary and especially permanent teeth) is that the resultant management and prognosis differs greatly between the two. Management of isolated ankylosis is definitively treated by extraction of the offending tooth while eruption failure from a genetic defect may result in no treatment at all or multiple single tooth osteotomies. A complete understanding of the clinical presentation and biological mechanisms underlying eruption and eruption disorders will facilitate the best diagnosis and patient management of difficult clinical cases.