Today is the 3rd annual International Day of Medical Physics and a time to celebrate the medical physicists at Duke University. Everyday – often behind the scenes – medical physicists fulfill a variety of roles that ensure the safe and effective delivery of radiation therapy and diagnostic imaging. That means they directly impact the health and lives of patients, from the colon cancer patient undergoing external beam radiation therapy to the retired grandfather having brachytherapy for prostate cancer to the mother of two getting her yearly mammogram to the five-year-old boy who fell off his bike and may have a broken arm. They are the experts in physics, which is central to radiation therapy for the treatment of cancer, and the development and advancement of medical imaging techniques.