Monoclonal gammopathy of undetermined significance (MGUS) is a precancerosis comprising two different kinds of cancer: lymphoid/lymphoplasmocytoid MGUS and plasma cell MGUS that represents about 85% of all MGUS cases. This type of MGUS has low but persistent tendency to transform to malignant disease, mainly multiple myeloma (MM), with frequency of about 1% per year. Using known risk stratification models based on clinical parameters, it is possible to identify patients' groups with average rates of progression as low as 0.26% and as high as 12% per year. However, due to the lack of clear genetic and/or phenotypic markers distinguishing MGUS from MM, we are not able to predict if and when MGUS will progress to MM in individual patients. There are partially overlapping molecular pathogenic events shared by MGUS and MM. Better understanding of pathogenesis of MGUS and MM using molecular-genetic approaches will help disclose the mechanisms of myeloma genesis; it can be also useful for identification of novel molecular targets. The ultimate goal for the near future is to develop better markers for definition of high-risk MGUS patients who will be candidates for early treatment intervention.