Advances in computing and displays have enabled a recent surge in low-cost mixed-reality devices, including augmented reality (AR) and virtual reality (VR). These new capabilities have resulted in substantial interest in the development of medical extended reality (MXR) devices. Clinical researchers and medical device developers have begun to leverage this nascent technology for surgery and interventional procedures, diagnostics, and therapeutics. While these applications present new opportunities, the adoption, impact, and longevity of MXR applications in medicine hinges upon the development of evaluation techniques, quantitative assessment of the effectiveness, and a clear understanding of the value added by MXR over current clinical practice standards.