Multiple myeloma (MM) is a clonal plasma cell dyscrasia and the most common form of primary bone marrow cancer. Nearly 35,000 new cases of MM are diagnosed in the United States each year. MM is a slowly progressive illness that remains incurable. The median survival for patients with MM is approximately 7 years, during which these patients suffer substantial morbidity. Despite the introduction of new drugs and immune-based therapies, many patients unfortunately relapse and require further therapies. Therefore, it is becoming increasingly important to be able to accurately and quickly determine changes in a patient's clinical status. Assessments of monoclonal protein and serum free light chain levels are the most common tests now available for monitoring patients with MM; however, these assays have several drawbacks. Modern radiologic techniques such as positron emission tomography and computed tomography are better than standard radiographs but are costly and cumbersome. Serum B-cell maturation antigen is a new biomarker for both the diagnosis and prognosis of MM. Assessment of measurable residual disease is becoming an important endpoint. The creation of better ways to predict outcomes and promptly and accurately monitor changes for patients with MM should lead to improved quality of life and longer survival.