Now in its third edition the International Classification of Diseases for Oncology (ICD-O) has been used for nearly 25 years as the standard tool for coding diagnoses of neoplasms in tumour and cancer registrars and in pathology laboratories. ICD-O is a dual classification with coding systems for both topography and morphology. The topography code describes the site of origin of the neoplasm and uses the same 3-character and 4-character categories as in the neoplasm section of Chapter II ICD-10. The morphology code describes the characteristics of the tumor itself including its cell type and biologic activity. In preparing this new edition the editors have made a special effort to change as few terms as possible to add new terms at empty spaces and to avoid reuse of previously assigned codes. While all topography codes remain the same, as in the previous edition, morphology codes have been thoroughly reviewed and where necessary revised to increase their diagnostic precision and prognostic value. Most changes reflect the urgent need to code new diagnoses in haematopathology. These changes have been made in line with improved understanding of the pathogenesis and clinical behavior of several types of leukaemia and lymphoma. The book has five main sections. The first provides general instructions for using the coding systems and gives rules for their implementation in tumor registries and pathology laboratories. Section two includes the numerical list of topography codes, which remain unchanged from the previous edition. The numerical list of morphology codes is presented in the next section which introduces several new terms and includes considerable revisions of the non-Hodgkin lymphoma and leukemia sections based on the WHO Classification of Hematopoietic and Lympoid Diseases. The five-digit morphology codes allow identification of a tumor or cell type by histology behavior and grade. Revisions in the morphology section were made in consultation with a large number of experts and were finalized after field-testing in cancer registries around the world. The alphabetical index produced in section four gives codes for both topography and morphology and includes selected tumor-like lesions and conditions. A guide to differences in morphology codes between the second and third editions is provided in the final section which includes lists of all new code numbers new terms and synonyms added to existing code definitions terms that changed morphology code terms for conditions now considered malignant deleted terms and terms that changed behavior code. To the greatest extent possible ICD-O uses the nomenclature published in the WHO series International Histological Classification of Tumours.