Of the many methods described in this paper only a few have been validated. The burning and cracking method has been tested on a number of occasions, and has in most cases been found suitable. The different grinding and sliceing techniques have not been tested so extensively, but the studies of Liew (1974), Berg (1985), Tulonen (1987) and Wickstrøm (1987) indicate that also these methods may be useful and valid. The clearing method have been found useful in some cases (Vøllestad and Jonsson, 1988; Lecomte-Finiger et al., in prep.) but inappropriate for very slow growing eels (Vøllestad and Næsje, 1988). Burning and cracking thus seems to be the best method available, it is therefore proposed that this method be used as a reference method for further studies. It should perhaps be stressed that any technique should be tested anew when studies are started on new populations in new habitats. This was also pointed out by Beamish and McFarlane (1983), who emphasized that estimates of age are difficult to produce and that they are not unequivocal. But because such estimates are fundamental to our understanding of the biology of fishes and of the dynamics of fish populations it is essential that they be proven accurate.