Multiple myeloma（MM） lesions are mostly localized in the marrow. Extramedullary disease in multiple myeloma （MM-EMD） is defined as malignant plasma cell infiltration away from the bone marrow or adjacent soft tissue， may occur at the initial diagnosis or during the consultation. MM-EMD may be found at initial diagnosis or during the treatment. MM-EMD has high invasiveness and poor prognosis， with clinical behavior distinct from marrow-restricted myeloma. However， its pathogenesis has not been elucidated. In general， the obstructed homing of myeloma cells， enhanced invasiveness， the degradation of extracellular matrix， and increased angiogenesis capacity may be involved in the occurrence of MM-EMD. Tumor genetic abnormalities and changes in the bone marrow microenvironment play important roles in the above pathogenesis.