Abstract Objective.—This article summarizes the most useful ancillary immunohistochemical and molecular assays for use in the diagnosis of mantle cell lymphoma. Data Sources.—The English language literature was surveyed, with an emphasis on recent publications, for articles presenting key advances in the molecular characterization of mantle cell lymphomas and for series of cases testing the utility of molecular diagnostic tests. The authors' series of 26 small B-cell lymphomas, analyzed for the cyclin D1 protein by paraffin immunohistochemistry and for t(11;14) by polymerase chain reaction, is included. Conclusions.—Mantle cell lymphoma, a B-cell lymphoma now recognized in the 1994 Revised European-American Classification of Lymphoid Neoplasms (REAL) classification, is a relatively aggressive lymphoma with a poor prognosis. Its characteristic t(11;14)(q13;q32) translocation has a role in oncogenesis and has been exploited for molecular diagnostic tests, but these tests vary in sensitivity, specificity, and ease of use. Improved immunohistochemical tests are sufficient to confirm the diagnosis in most cases. Conventional cytogenetics and molecular diagnostic tests for t(11;14)—Southern blot and polymerase chain reaction analysis—may be helpful in selected cases, but are laborious or of limited sensitivity. Other methods, such as fluorescence in situ hybridization, need further development to provide faster, more sensitive diagnosis.