Extramedullary disease of multiple myeloma (EM) remains a treatment challenge even in the era of new drugs. While many reports analyzing various aspects of EM have been published, mechanism of EM development has not been clarified yet. This review summarizes current knowledge about this clinical entity, including its history, diagnostics, imaging methods, incidence, prognosis, current treatment options, risk factors and known molecular mechanisms that might be involved in pathogenesis of EM.