Smoldering multiple myeloma (SMM) consists of the presence of a serum M protein of 30 g/L or more and/or 10% or more bone marrow plasma cells (BMPCs), with no clinical manifestations or symptoms of myeloma. It accounts for approximately 10% of all myelomas, and the median time to progression to a symptomatic multiple myeloma ranges from 2 to 3 years. The main factors for progression are the plasma cell mass (M-protein size and percent of BMPCs), the spinal MRI pattern, the plasma cell proliferative index, and the variant of SMM ("evolving" vs "nonevolving"). Although treatment with thalidomide is promising (based on the results of two phase II trials), outside the context of a clinical trial, a watch-and-wait approach with clinical evaluation every 4 months is recommended until evident symptomatic disease progression occurs. Patients with monoclonal gammopathy of undetermined significance (MGUS) have a serum M protein lower than 30 g/L and a proportion of BMPCs of less than 10%, with no clinical findings or symptoms attributable to the monoclonal gammopathy. MGUS has a high prevalence, and its annual rate of malignant transformation is 1%, such that the actuarial probability of progression to a symptomatic monoclonal gammopathy at 25 years of follow-up is as high as 40%. The factors associated with a higher probability of malignant transformation are a relatively high plasma cell mass, immunoglobulin A M-protein type, and the "evolving" variant. It is recommended that patients with MGUS are monitored annually. Importantly, patients with asymptomatic monoclonal gammopathies must not be treated before the development of overt multiple myeloma.