To evaluate, interpret, and better communicate the data and findings from long-term chemical carcinogenesis studies on laboratory animals, the National Toxicology Program began using five categories or levels of evidence of carcinogenicity in 1983 (clear, some, equivocal, and no evidence and inadequate experiment). Through July 1991 these defined terms had been used to describe 144 chemical carcinogenesis studies comprising 530 sex-species experiments. Together with a selected descriptor of the chemically associated level of evidence for each experimental unit (male rats, female rats, male mice, female mice), mention is made of the length of the experiment, route of exposure, and the particular tumor type or types for each organ or system affected. The scientific judgements are comprised of this relevant information to inform the readers and users of these evaluations. In this paper the background rationale for the development and proper use of these categories of evidence of carcinogenicity are detailed, together with some personal reflections.