Magnetic resonance imaging (MRI) is a technique that uses a magnetic field and radio waves to create detailed images of the organs and tissues within your body. MRI is safe, but if something goes wrong, it can go very wrong. Most reported cases of MRI-related injuries and the few fatalities that have occurred have apparently been the result of failure to follow safety guidelines or of use of inappropriate or outdated information related to the safety aspects of biomedical implants and devices. To prevent accidents in the MRI environment, therefore, it is necessary to revise information on biologic effects and safety according to changes that have occurred in MRI technology and with regard to current guidelines for biomedical implants and devices. This review provides an overview of and update on MRI biologic effects, discusses new or controversial MRI safety topics and issues, and presents evidence-based guidelines to ensure safety for patients and staff.