The class II genes of the major histocompatibility complex (MHC) encode the alpha/beta heterodimeric glycoproteins that play a critical role in the induction of immune responses through presentation of processed antigen to CD4+ T lymphocytes. The constitutive expression of class II MHC antigens is restricted primarily to B cells, dendritic cells, thymic epithelium, and macrophages, although a wide variety of other cell types can be induced to express class II antigens after exposure to cytokines. The appropriate constitutive and inducible te constitutive and inducible expression of class II MHC antigens is essential for normal immune function; thus it is not surprising that aberrant expression on cell types normally class II MHC negative has been correlated with various autoimmune disorders, and lack of expression results in a severe combined immunodeficiency disorder called bare lymphocyte syndrome (BLS). In this review, we discuss the agents that both induce and inhibit class II MHC expression, the function of class II MHC antigens with an emphasis on the ability of these proteins to act as signal transducing molecules, and the molecular regulation of class II MHC expression.