The WHO separates marginal zone B-cell lymphomas (MZBL) of nodal and extranodal type. Both arise from memory B cells of the marginal zone. Extranodal MZBL of the mucosa-associated lymphatic tissue (MALT) have characteristic features such as homing in epithelial tissue, lymphoepithelial destruction, and a very indolent clinical course. They arise in epithelial tissues normally devoid of lymphatic cells, where the lymphatic tissue was acquired after, for instance, a chronic infection. The best example here is infection of the stomach with Helicobacter pylori ( Hp). Besides the classical association with gastric MALT lymphomas, there have been reports in which an association between Hp and diffuse large B-cell lymphoma (DLBCL) has been observed as well. Consequently, cure of Hp infection resulted in remission induction not only in gastric MALT lymphomas, but also in some patients with very limited stages of DLBCL of the stomach. In addition to the association with Hp, progress has been made with regard to MALT lymphoma biology. Translocation t(1;14) involving the Bcl-10 gene, and translocation t(11;18) involving a novel gene called MLT1, both result in activation of the crucial transcription factor NF-kappaB. These genetic events seem specific in that they have been observed only in MALT lymphomas. Once present, at least the more frequently observed translocation t(11;18) renders cells resistant to cure of Hp infection. Another clinically important question is that in many patients in complete remission after cure of Hp infection, detection of minimal residual disease is positive. Whether these cells are normal memory B cells (with the identical B-cell rearrangement as the original lymphoma clone), or dormant lymphoma cells, is unclear at present. In patients not responding to cure of Hp infection, several treatment options are discussed. MALT lymphomas have opened up a new discussion of lymphoma biology and have thus been called a model disease.