A workshop on age determination in harbour porpoises (Phocoena phocoena) was held in Oslo, Norway, 21-23 May 1990. During the workshop, the principles of formation of dental layers were reviewed, as were the growth layer patterns in odontocete teeth and evidence for interpreting the layering patterns as annual. Specific information on how to estimate age in harbour porpoises, identify anomalous mineralisation patterns and use the teeth for discriminating stocks was also presented. An experiment was conducted to test for within- and between-reader variability in age estimates in a sample of teeth provided at the workshop. The teeth were read by three people with experience and four people with little or no experience in harbour porpoise age estimation. An analysis of variance of the experimental results showed more variation for inexperienced readers than for experienced and more for older animals than younger. For the experienced readers, the reliability of age reading for animals less than five years of age was high. The pooled standard deviation was in this case 0.32yrs, which was made up of contributions from pure error, between-section and between-reader variability, in increasing order of importance. This report includes all presentations, results of the experiment and a series of annotated photos of harbour porpoise tooth sections.