Prognostic factors in oncology may help physicians give their patients a prognosis for their disease and thus allow them to make plans for the future. These factors also assist in the selection of patients more likely to benefit from intensive treatments, especially in the context of clinical trials. Recently it has become clear that prognostic factors may have an additional benefit: they may provide insight into the biology of the cancer being studied and lead to improved understanding of the molecular pathogenesis. For malignant mesothelioma, the prognostic scoring systems of the Cancer and Leukemia Group B (CALGB) and European Organization for Research and Treatment of Cancer (EORTC) are the most useful of those currently available. These systems rate performance status, age, histological subtype, weight loss, and hematological parameters as the best prognostic factors for malignant mesothelioma. In the future, biological markers may provide additional information on mesothelioma and will help in prognostication.