Measurable residual disease (MRD) is an important indicator of disease progression and prognosis. Here, Stephan Grupp, MD, PhD, of the Children’s Hospital of Philadelphia, Philadelphia, PA, explains how MRD is used in pediatric and adult acute lymphoblastic leukemia (ALL). In pediatric ALL, MRD may be used as a signal to increase treatment intensity, and Dr Grupp highlights how MRD is currently being used in clinical trials to identify which patients should receive first-line CAR T-cell therapy. In adults, the relationship between treatment planning and MRD status is less well defined. Dr Grupp discusses the different MRD measurement techniques, including flow cytometry and PCR, suggesting that most clinicians will move towards next-generation sequencing as their preferred detection method in the future. This video was recorded at the American Society of Oncology (ASCO) 2018 Annual Meeting, held in Chicago, IL.