The monoclonal gammopathies are characterized by clonal proliferation of plasma cells and other clonally related cells in the B-cell lineage. These disorders include monoclonal gammopathy of undetermined significance, multiple myeloma, Waldenström macroglobulinemia, heavy chain diseases, plasmacytoma, and primary amyloidosis. Many skin disorders have been described in association with monoclonal gammopathies. This article provides an introduction to the definition, detection, natural course, and spectrum of monoclonal gammopathies and a brief discussion of pathogenesis. The article also reviews the skin disorders associated with monoclonal gammopathies, categorizes the association, and evaluates the strength of the association. (J Am Acad Dermatol 1999;40:507-35.) Learning objective: At the conclusion of this learning activity, participants should be familiar with the clinical spectrum of monoclonal gammopathies, the natural course of these disorders, and the skin disorders associated with them.