A number of methods for categorization of carcinogens, including classification and ranking, have been examined and applied to 13 carcinogenic chemicals. Compounds known to be carcinogenic to humans ranked highly in the ranking systems. The TD50 approach was a satisfactory ranking system. The examination of the classification system of the International Agency for Research on Cancer, and the studies of the ranking systems revealed the importance of considering the underlying mechanisms of carcinogenesis. Some of the important problems of categorizing carcinogens are discussed.