A monoclonal spike on serum protein electrophoresis is a frequent finding in the general population and pathognomonic of a plasma cell dyscrasia. In otherwise healthy individuals, it is diagnostic of two asymptomatic, premalignant conditions called monoclonal gammopathy of undetermined significance (MGUS) and smoldering multiple myeloma (SMM) which carry a lifelong risk of progression to multiple myeloma (MM) or related malignancy. This article discusses the criteria for diagnosis of MGUS, SMM, and MM; current recommendations for follow-up and risk factors for progression to MM of patients with MGUS and SMM; and diagnostic evaluation of suspected MM transformation.