Variation in zonation and Growth Layer Group (GLG) formation and underlying causes are reviewed. Five different types of anomaly were investigated in the teeth of harbour porpoise (Phocoena phocoena): pulp stones, mineralisation interference, marker lines, dentinal resorption and cemental disturbance. Samples of approximately 50 teeth from British Isles, Denmark, Canada, Norway and California were studied and findings compared. The incidence of all anomalies increased with age through natural persistence in the teeth. No differences in incidence of anomalies were found between the sexes, but significant differences in incidence by region for four anomaly types (excluding pulp stones) were observed. Similarities were seen between teeth from the British Isles and Denmark, and between Canada and Norway, which could not be explained by geographical proximity. Various hypotheses concerning environmental factors, including pollution and life history events, were discussed as the possible causes of these anomalies and their different levels of incidence. The most promising result appears to be a possible link between marker lines and life history events, a hypothesis which is open to experimental testing, with the use of tetracycline time-marking techniques of teeth and known-history captive animals.