Monoclonal gammopathy of unknown significance (MGUS) as one of the most common premalignant disorders and smoldering multiple myeloma (sMM) are both caused by a proliferation of monoclonal plasma cells leading to a detectable serum monoclonal protein and/or excess of plasma cells in the bone marrow. Prerequisite for the diagnosis is that plasma cell disease does not cause clinical symptoms. Cytogenetic aberrations are detectable in the majority of patient in the clonally expanded plasma cells. MGUS consistently proceeds symptomatic MM. The lifetime risk of progression into symptomatic multiple myeloma lies between 15% and 59% for patients with MGUS or sMM. Prognostic parameters for development of symptomatic multiple myeloma from MGUS or sMM are concentration of monoclonal protein, bone marrow plasmocytosis, a non- IgG subtype and an abnormal free-light chain ratio. Detection of more than 1 focal lesion in whole body MRI, 95% or more of bone marrow plasma cells displaying an aberrant phenotype in flow cytometry and an evolving clinical course in two consecutive follow-up visits are additional prognostic parameters for sMM. Currently there is no accepted secondary prevention strategy available for sMM and MGUS progression. Future studies are required to combine increasing knowledge on risk factors and molecular pathogenesis with targeted agents to prevent progression.