The diagnosis of extranodal malignant lymphomas, as well as their distinction from extranodal lymphoid hyperplasias, depends on the evaluation of specific histopathologic criteria that were described originally for lymph nodes. The major criteria traditionally include architectural effacement, cellular monomorphism, and cytologic atypia. Although these criteria may be applied directly to extranodal lymphoid lesions, there are limitations that may lead to morphologic ambiguity. Part of the problem may be technical, but difficulties also ensue because benign reactive lymphoid hyperplasias may coexist with extranodal lymphomas, and because extranodal lymphomas may be focal, exhibit cellular polymorphism, and be cytologically mature. Awareness of the histologic exceptions, combined with prudent application of the conventional criteria, should improve the accuracy of diagnosis of extranodal lymphocytic infiltrates.