Mucosa-associated lymphoid tissue (MALT) lymphomas are a distinct subgroup of non-Hodgkin's lymphoma with a particular clinicopathologic behavior. The gastrointestinal tract is involved in two thirds of the cases, but it may be observed in lung, breast, bladder, conjunctiva, kidney, liver, skin, salivary glands, thyroid, and thymus. This type of lymphoma tends to appear in patients with a history of autoimmune disease or chronic inflammatory disorders. In the stomach, this lesion is induced by Helicobacter pylori and is characterized by an accumulation of lymphoid tissue leading to chronic gastritis. The preceding lymphoid disorder is represented by Sjögren's syndrome, Hashimoto thyroiditis, and interstitial lymphoid pneumonia in the thyroid, salivary glands, or lung, respectively. Lymphoma cells initially arise from the marginal zone localized around reactive follicles and secondarily invade epithelial tissue to form the characteristic lymphoepithelial lesion. Patients with gastrointestinal MALT lymphoma generally present with localized disease without any adverse prognostic factors. These patients have long survival rates. Recurrences may appear in the same organ or in other extranodal sites. Nongastrointestinal MALT lymphoma patients seem to have a similar outcome. Patients may be treated with surgery or radiotherapy if the disease is localized, or with single-agent chemotherapy if it is disseminated. Reversion of the chronic inflammatory disorder with antibiotics, such as for gastric involvement, is a new observation that may change therapeutic options in the future.