Monoclonal gammopathy of undetermined significance (MGUS) is an asymptomatic disorder with serum monoclonal immunoglobulin less than 30 g/l. It preferentially affects elderly patients; MGUS prevalence is about 1% in the general population and about 10% in patients aged over 80 years. MGUS diagnosis is based on elimination of other causes of serum monoclonal immunoglobulin, particularly multiple myeloma. Within the 20 years following diagnosis of MGUS, about 25% of patients will evolve towards myeloma or other malignant lymphoproliferative disorder. No factor has been identified to date which can efficiently predict this evolution. Recent data concerning immunophenotype, cytogenetics and molecular biology of plasma cells demonstrate the link between MGUS and multiple myeloma. MGUS clearly appears now as a plasma cell monoclonal pathology with reduced malignity. Soon, new biological data would help to discriminate patients with MGUS who will remain asymptomatic life-long from those who will evolve towards malignant lymphoproliferative disorder.