Non-Hodgkin lymphoma is a non-specific term that includes several lymphoproliferative malignant diseases with different clinical and histological appearances. Here, we concentrate on adult lymphomas. We look at their molecular basis, at the development of a classification system based on a better understanding of the biology of the various subgroups, and at how refinement of adverse prognostic factor groupings helps in clinical management. Lymphomas can present in various ways and be difficult to diagnose. About a quarter of cases arise extranodally and might present special problems. Developments in cytotoxic chemotherapy have led to good long-term survival prospects for aggressive lymphoma; introduction of novel approaches, including monoclonal antibody therapy, offers promise for indolent lymphoma, and should further improve prognosis for aggressive tumours.