Abstract Increasing therapeutic options and prolonged survival in multiple myeloma have raised interest in the concept of depth of response and its importance to predict patients' outcomes. Although the efficacy of current treatment approaches has greatly improved in the past decade, the definition of complete response (CR) remains unaltered and continues to use conventional serological and morphologic techniques. That notwithstanding, there is growing interest in minimal residual disease (MRD) monitoring, which has emerged in recent years as one of the most relevant prognostic factors in multiple myeloma. MRD can be assessed both inside (e.g., immunophenotypic and molecular techniques) and outside the bone marrow (e.g., PET/CT). Here, we focus on flow- and molecular-based assays by which different cooperative groups have demonstrated the efficacy of MRD assessment to predict outcomes even among patients in CR, and irrespectively of disease risk. Although further standardization is still required, the time has come to implement MRD monitoring in prospective clinical trials as a sensitive tool to evaluate treatment efficacy and for risk-adapted treatment, particularly in the consolidation and maintenance settings. Here, we present a comprehensive and critical review on the methodologic aspects, specific characteristics, and clinical significance of MRD monitoring by flow cytometry, PCR, and next-generation sequencing. Clin Cancer Res; 21(9); 2001–8. ©2015 AACR.