The non-Hodgkin's lymphomas (NHLs) comprise a heterogeneous collection of lymphoproliferative malignancies, which are most common in people aged over 55 years. Diffuse large B-cell lymphoma (DLBCL) is the most common type of NHL, accounting for approximately 30% of all new patients. Follicular lymphoma (FL) is the second most common NHL sub-type, and accounts for a further 22% of cases. While the incidence of most other cancers is decreasing, that of NHL is increasing steadily. During the 1970's and 1980's, worldwide NHL incidence rose by 3-4% per year. This rise has slowed in the 1990's, but an annual increase of 1-2% is still being recorded. Over the last five years, the introduction of monoclonal antibodies, and specifically the anti-CD20 monoclonal antibody, rituximab, has radically changed treatment of B-cell NHL. Rituximab is a genetically engineered chimeric mouse/human monoclonal antibody which binds to the transmembrane antigen, CD20, a non-glycosylated phosphoprotein, located on pre-B and mature B lymphocytes. This antigen is expressed on over 95% of all B cell NHLs, and on normal B cells, but not on haematopoietic stem cells, normal or malignant plasma cells. The Fc domain of rituximab recruits immune effector functions to mediate B cell lysis. Possible mechanisms of cytotoxicity include complement-dependent cytotoxicity (CDC) resulting from C1q binding, and antibody-dependent cellular cytotoxicity (ADCC) mediated by one or more of the Fcgamma receptors on the surface of granulocytes, macrophages and NK cells. It is also possible that the binding of rituximab to the CD20 antigen on the cell surface may directly induce apoptosis. For patients with both follicular and diffuse large B-cell NHL, several large scale prospective randomised trials have demonstrated prolongation of remission when rituximab is incorporated into first line treatment, and, in follicular lymphoma, as a component of salvage therapy. As a result of these studies, current European indications for rituximab include: the treatment of previously untreated patients with stage III-IV follicular lymphoma in combination with cyclophosphamide, vincristine and prednisone (CVP) chemotherapy; as maintenance therapy in patients with relapsed follicular lymphoma responding to induction therapy with chemotherapy or immuno-chemotherapy; the treatment of patients with diffuse large B cell NHL in combination with cyclophosphamide, doxorubicin, vincristine and prednisone (CHOP) chemotherapy. This paper examines the evidence supporting the use of rituximab in these settings, and places its use into the context of standard clinical practice.