The objective of long-term carcinogenicity studies is to observe test animals for a major portion of their lifespan for the development of neoplastic lesions during or after exposure to various doses of a test substance given by an appropriate route. Studies of this type require careful planning and documentation of the experimental design, a high standard of pathology, good animal care and suitable statistical analysis. These requirements are well known and have not undergone major changes during recent years. Rather than giving details of these requirements, which can now be found in many publications, this paper attempts to indicate some of the problems involved in a long-term carcinogenicity study.