Regulatory authorities worldwide still depend greatly upon and require long-term animal test results. To improve the reliability of interpretation of such results, a standardized nomenclature for the lesions observed in the tests is essential. Scientists from both academia and industry in many countries have closely cooperated to arrive at a consensus on the descriptions of all the types of tumour and pre-neoplastic lesions encountered in laboratory mice. The series of fascicles should provide information and guidelines especially adapted for international use in practical toxicologic pathology. Images showing the typical appearance of the discussed lesions and references to the most recently published papers complete the presented information.