Marginal zone lymphomas (MZLs) comprise 3 distinct entities: extranodal MZL of mucosa-associated lymphoid tissue (MALT), splenic MZL, and nodal MZL. Gastric MALT lymphoma is the most common extranodal MZL and often develops as a result of chronic Helicobacter pylori gastritis. Such cases frequently respond to antibiotics directed against H. pylori. Antigen-driven lymphomatous disease can also be seen in the association of Borrelia burgdorferi with MALT lymphoma of the skin, Chlamydia psittaci with MALT lymphoma of the ocular adnexa, Campylobacter jejuni with immunoproliferative disease of the small intestine, and hepatitis C with splenic MZL. This article discusses the pathogenesis and clinical features of MZL and the treatment options available to patients.