Criteria for the diagnosis of low-grade B-cell lymphoma of mucosa-associated lymphoid tissue (MALT) type differed among participants in the Society for Hematopathology Workshop. Some would accept any extranodal lymphoma that lacked features of another lymphoma type as MALT type, while others required the presence of all morphologic criteria. Criteria for the diagnosis of MALT-type lymphoma can be clinical, morphologic, and biologic, and the implications of making the diagnosis may dictate how restrictive the criteria for diagnosis should be. The morphologic definition of MALT lymphoma is a lymphoma that recapitulates the structure of the Peyer's patch. The clinical definition is an indolent extranodal lymphoma that tends to remain localized or to recur in other extranodal sites, with a potential for cure with local therapy. The biologic definition is unknown, but it includes an antigen-stimulated B cell with a special relationship to epithelial tissues and germinal centers. For clinical purposes, exclusion of one of the other types of small B-cell lymphoma may be sufficient to dictate treatment. For investigation and further understanding of lymphomas, however, a more restrictive definition should be used, since there may be other small B-cell lymphomas not yet described.