Summary The non-Hodgkin's lymphomas (NHL) are a heterogenous group of disorders characterised by malignant proliferation of lymphoid cells. The cellular origin is relatively well established with subtypes corresponding to the various stages of lymphocyte differentiation. The term encompasses a hotchpotch of conditions with very different morphological appearance, behaviour and clinical outcome. NHL comprise 2.4% of all cancers, with incidence increasing with age. The commonest presentation is with progressive lymphadenopathy, though extranodal manifestations are present in a significant proportion. The clinical behaviour ranges from a benign, indolent course to rapidly progressive disease; prognosis varies from weeks to many years. Treatment is correspondingly diverse, from ‘watchful waiting’ to high-dose chemotherapy with bone marrow stem cell transplantation. Cure is possible in an increasing number of patients and much interest currently lies in identifying patients with high-risk disease necessitating the use of intensive treatment regimens.