Lifetime assays for carcinogenicity in experimental animals, together with data on genetic and related effects on a variety of organisms, have long been the foundation for predictions of carcinogenic hazard to human beings. Recent scientific advances have provided new assays and novel test systems that are beginning to supplement, and in the future may even replace, the well-established tests that have been widely used for the last four decades. This publication reviews the evidence that justifies the use of lesions that precede histologically defined malignancy as endpoints to predict carcinogenicity. It evaluates the utility of non-mammalian species and of genetically engineered rodents as subjects for carcinogenicity tests and mutations in cancer-related genes in human and experimental animal tumours as 'footprints' of environmental carcinogens. It evaluates the use of established and novel assays for genetic toxicity in the prediction of carcinogenicity. Finally it formulates recommendations on the use of such data in the process of evaluation of carcinogenic hazards by the IARC Monographs on the Evaluation of Carcinogenic Risks to Humans.