Past attempts to age classify dog-whelks are revised briefly. The most satisfactory method is a system which relies upon familarity of morphological characteristics with which to separate population samples into five different classes. An attempt has been made to quantitate this system. Definite standards have been suggested as the criteria with which to separate the five age classes, and a series of simple methods have been demonstrated which, when followed give finite values for each of these standards. The standards suggested are: (i) the lower limits for shell-length in mature individuals; (ii) the lip-thickness coinciding with maturity; (iii) the upper limits for shell-length in juvenile crevice dwellers. The methods of estimate for values for these standards are illustrated in practice with population samples from Blackrock, Sussex. The values thus obtained and the methods used in each estimation are discussed with view to their validity. It is reasonable to suspect that the quantitative age classification system can be used to classify dog-whelks from any shore provided large enough population samples are collected to estimate the values for each standard. Use of the system is further demonstrated by classification of animals from the shore at Martin's Haven, Pembrokeshire.