Primary gastric lymphoma is the second most common tumour of the stomach after adenocarcinoma. Diffuse large B-cell lymphoma (DLBCL) is the most common form of primary gastric lymphoma, closely followed by marginal zone lymphoma of mucosa-associated lymphoid tissue (MALT). Low-grade MALT lymphoma is often multifocal in the stomach, develops in association with Helicobacter pylori infection and is composed of small marginal-zone B cells recapitulating the features of MALT. Eradication of H. pylori leads to complete regression of MALT lymphoma in up to 80% of patients not harbouring the t(11;18) translocation. The differential diagnosis of gastric MALT lymphomas includes reactive lymphoid hyperplasia (H. pylori gastritis) and other small B-cell lymphomas involving the stomach, usually as a secondary manifestation of a systemic disease. Gastric DLBCL occurs anew or as a transformation of t(11;18)-negative MALT lymphoma; it is morphologically similar to nodal DLBCL, but the associated genetic alterations are different. It is primarily treated by polychemotherapy and immunotherapy, supplemented by antibiotics in H. pylori-positive cases. Mantle-cell lymphoma, Burkitt's lymphoma and peripheral T-cell lymphoma represent other, less common, forms of primary gastric lymphoma. Lymphoproliferative disease, arising in a context of immune deficiency, may also present as primary gastric lesions.