The major histocompatibility complex (MHC) class II genes encode surface molecules that are required for presentation of antigenic peptides to helper T-cells. The concentration of these proteins on the surface of effector cells (antigen-presenting cells such as B-cells and macrophage) is one of the parameters affecting the intensity of the immune response. Many studies have thus focused their attention on the mechanisms that control the expression of class II genes, particularly in B-cells. The anatomy of MHC class II promoters has been dissected in detail, and many trans-acting factors and their cognate DNA regulatory elements have been identified and characterized, thus helping to elucidate the molecular circuitry which determines tissue-specific, coordinate expression of these genes. In most cases, regulation has been investigated at the level of mRNA transcription. MHC class II gene expression has been observed as well, under physiological conditions, in many other tissues and organs such as brain, thyroid, thymus, and intestine, thus implying that class II molecules may be involved, whether directly or indirectly, in the modulation of other important biological responses in addition to the control of the immune reaction against soluble antigens. Spurious MHC class II activity is also detected in tumor cells and in other pathological conditions such as those found in autoimmune, inflammatory, and infectious diseases. In autoimmunity, cells that express class II molecules may present tissue-specific antigens, thus triggering a mechanism of self-destruction. In tumors, instead, unscheduled MHC class II expression may be part of a mechanism that prevents tumor progression. Comprehension of the regulatory functions operating in pathological conditions as compared to those active in B-cells and in macrophages is still rudimentary. Because of the possible pathogenetic importance of aberrant class II expression, knowledge of the cis-and trans-acting elements controlling gene expression at either the transcriptional or posttranscriptional level may allow the development of strategies for immunointervention against these diseases.