Planning for and exercising the medical response to potential chemical, biological, radiological, nuclear, and explosive (CBRNE) terrorist events are new responsibilities for most health care providers. Among potential CBRNE events, radiological and/or nuclear (rad/nuc) events are thought to have received the least attention from health care providers and planners. To assist clinicians, the U.S. Department of Health and Human Services (HHS) has created a new, innovative tool kit, the Radiation Event Medical Management (REMM) web portal (http://remm.nlm.gov). Goals of REMM include providing (1) algorithm-style, evidence-based, guidance about clinical diagnosis and treatment during mass casualty rad/nuc events; (2) just-in-time, peer-reviewed, usable information supported by sufficient background material and context to make complex diagnosis and management issues understandable to those without formal radiation medicine expertise; (3) a zip-file of complete web portal files downloadable in advance so the site would be available offline without an Internet connection; (4) a concise collection of the printable, key documents that can be taken into the field during an event; (5) a framework for medical teams and individuals to initiate rad/nuc planning and training; and (6) an extensive bibliography of key, peer-reviewed, and official guidance documents relevant to rad/nuc responses. Since its launch, REMM has been well received by individual responders and teams across the country and internationally. It has been accessed extensively, particularly during training exercises. Regular content updates and addition of new features are ongoing. The article reviews the development of REMM and some of its key content areas, features, and plans for future development.