Minimal residual disease (MRD) has become a central concept in modern oncology, reshaping how clinicians evaluate response, relapse risk and treatment precision. As increasingly sensitive technologies reveal traces of cancer that persist after therapy, MRD is emerging as both a biological challenge and a clinical opportunity, especially as new data illuminate its complexity across hematologic and solid tumors. This topic was addressed at the 2026 American Association for Cancer Research (AACR) annual meeting.