This paper is an account of preparation and examination techniques and criteria used to estimate age in decalcified and stained tooth thin sections from spinner and spotted dolphins. A dentinal growth layer group (GLG),composed of two thin light and two thicker dark-stained layers, is deposited annually. The GLG component layers are variably visible, but the "ideal" pattern and successive thinning of dentinal GLGs are used as a guide to determine GLG limits. Age-specific thicknesses of dentinal GLGs found in Hawaiian spinner dolphin teeth seem to be applicable to teeth of spotted dolphins and can be used as an aid in locating GLG boundaries. Cementa1 GLGs are composed of a dark-stained and alightly stained layer and usually aredeposited at a rate of one per year, but may be deposited every other year or two or three times per year. Two slightly different methods of counting dentinal GLGs are presented, along with guidelines for determining whetherdentinal or cementa1 GLG counts provide the best estimate of age for a specimen. (PDF contains 23 pages.)