AbstractDiagnosis, prognosis, and treatment are the three core elements of the art of medicine. Modern medicine pays more attention to diagnosis and treatment but prognosis has been a part of the practice of medicine much longer than diagnosis. Cancer is a heterogeneous group of disease characterized by growth, invasion and metastasis. To plan the management of an individual cancer patient, the fundamental knowledge base includes the site of origin of the cancer, its morphologic type, and the prognostic factors specific to that particular patient and cancer. Most prognostic factors literature describes those factors that directly relate to the tumor itself. However, many other factors, not directly related to the tumor, also affect the outcome. To comprehensively represent these factors we propose three broad groupings of prognostic factors: ‘tumor’‐related prognostic factors, ‘host’‐related prognostic factors, and ‘environment’‐related prognostic factors. Some prognostic factors are essential to decisions about the goals and choice treatment, while others are less relevant for these purposes. To guide the use of various prognostic factors we have proposed a grouping of factors based on their relevance in everyday practice; these comprise ‘essential,’ ‘additional,’ and ‘new and promising factors.’ The availability of a comprehensive classification of prognostic factors assures an ordered and deliberate approach to the subject and provide safeguard against skewed approaches that may ignore large parts of the field. The current attention to tumor factors has diminished the importance of ‘patient’ (i.e., ‘host’), and almost completely overshadows the importance of the ‘environment’. This ignores the fact that the latter presents the greatest potential for immediate impact. The acceptance of a generic prognostic factor classification would facilitate communication and education about this most important subject in oncology. Semin. Surg. Oncol. 21:13–18, 2003. © 2003 Wiley‐Liss, Inc.