The purpose of this article is to focus on aetiology and appropriate treatment techniques concerning anklyosis of primary molars.The dental literature is reviewed in detail concerning aetiology, frequency of occurrence, diagnosis and longevity of ankylosed primary molars without successors. Treatment concepts are discussed. Long term implications of treatment decisions made in the mixed dentition are emphasised. Areas of treatment that are unsupported by evidence are identified as potential research topics.When the underlying premolar is present and the infra-occlusion is not progressive, then observation is appropriate. Only when there is severe disruption to the occlusion and/or the underlying premolar, extraction and space management may be appropriate. When the ankylosed primary molar has no underlying premolar, orthodontic input is needed to determine if extraction and space closure, extraction and transplantation or extraction and prosthetic replacement is the best plan.