Purpose of Review <p>This review addresses the current treatment paradigm and new advancements in the management of microsatellite instability-high/mismatch repair deficient (MSI-H/dMMR) esophagogastric cancer (EGC).</p> Recent Findings <p>While chemotherapy and surgery remain the cornerstone of EGC treatment, MSI-H/dMMR tumors harbor high tumor mutational burden and represent a subset of patients who benefit from immune checkpoint inhibitors (ICI). ICI has been incorporated in the front line setting with and without chemotherapy for advanced disease. Recently, ICI has been studied in the perioperative setting for resectable disease. Though perioperative ICI results in improved response rates, it is not yet clear whether this translates to a survival benefit.</p> Summary <p>Despite high response rates with ICI in this patient population, many do not respond to therapy, representing a major challenge in treatment. Preclinical studies have highlighted potential mechanisms of resistance which will guide drug development and clinical trials.</p>