Scales, bones, fin rays and otoliths have all been used to determine the age of fish, since these and other bony parts of fish often form yearly rings (annuli) like those of a tree. However, otoliths generally provide the most accurate ages, due largely to their continued growth throughout the life of the fish and their acellular nature (which implies that they are not subject to resorption). These features give otoliths a significant advantage over scales and other structures, particularly in old fish. Therefore, otoliths have become the preferred structures for age determinations.

Methods for ageing sharks or skates and rays are analogous, but use vertebrae.

Despite the long-term global use of otolith annuli as age indicators, the factors influencing their formation is not clear cut. Each annulus is comprised of an opaque and translucent zone, which in many species correspond to fast and slow growth respectively. The opposite pattern is apparent in other species. In general though, the opaque zone seems to be formed during periods of increasing water temperatures, while the translucent zone is formed during periods of reduced growth, or in association with spawning.

The preferred method for otolith preparation and interpretation differs among species and age readers. In general however, microscopic examination of whole otoliths (immersed in a clear fluid) is acceptable for thin otoliths, while sectioning or the "crack and burn method" is required for other otoliths. Since otolith length may cease to grow in old fish, even while they continue to thicken, some form of cross section is required of old otoliths. The best papers on otolith preparation and annulus interpretation in temperate and tropical fish otoliths are presented in Chilton and Beamish (1982) and Fowler (1995) respectively.  Detailed methods on preparing thin sections are also presented elsewhere in this site.