Marginal zone lymphoma (MZL) is a distinct subgroup of non-Hodgkin's lymphoma (NHL), which is typically characterized by an indolent clinical course and long survival duration. MZL is responsible for approximately 7~8% of all NHL. In Korea, MZL accounts for 21% of all B-cell lymphoma and is the second most frequent histologic subtype following diffuse large B-cell lymphoma. Annually, an estimated 500 patients are newly diagnosed with MZL. According to the previous large-scale analyses, MZL is usually a quiet indolent malignancy, which generally presents with limited stage of disease. Localized disease may be controlled with local treatment, and a high response rate can be achieved. Advanced MZL is associated with less favorable survival, and appears to be incurable with the currently available therapy. Transformation to large cell aggressive lymphoma may occur in the first recurrence or in subsequent relapses.