After the first MRI scanners were approved by the FDA in 1984, they were rapidly adopted by the clinical community and have become one of the most common imaging tools. The MR environment produces unique safety requirements for implants and other medical devices. The principal MR safety issues are magnetically induced force and torque, RF heating, and image artifact. This article defines MR characteristics that affect safety and outlines the potential hazards produced by medical devices in the MR environment. It also summarizes ASTM International test methods that can be used to evaluate the MR safety of medical devices and describes ongoing efforts in MR standards development.