Fin spine ageing is the most common technique used to estimate age and growth parameters of large pelagic billfishes from the families Istiophoridae and Xiphiidae. The most suitable methods for processing and interpreting these calcified structures for age estimation have not been clearly defined. Methodological differences between unvalidated ageing studies are of particular concern for highly migratory species because multiple researchers in different regions of the world may conduct age estimates on the same species or stock. This review provides a critical overview of the methods used in previous fin spine ageing studies on billfishes and provides recommendations towards the development of a standardized protocol for estimating the age of striped marlin, Kajikia audax and white marlin, Kajikia albida. Three on-going fin spine ageing studies from Australia, Hawaii, and Florida are used to illustrate some of the considerations and difficulties encountered when developing an ageing protocol for highly migratory fish species. Particular areas of concern that may influence age and growth estimates included differences in fin spine selection, sectioning methods, criteria for identifying and measuring annuli, distinguishing false annuli, validation procedures, identification of the first annulus, and methods used to replace annuli lost due to vascularization of the fin spine core.