Diagnostic reference levels (DRLs) are used in medical imaging to indicate whether the patient radiation dose or amount of administered activity from a specific procedure are unusually high or low for that procedure. DRLs are the first step in the optimization process to manage patient dose commensurate with the medical purpose of the procedure. Achievable dose is an optimization goal, based on survey data, and typically defined as the median value (50th percentile) of the dose distribution of standard techniques and technologies in widespread use. The overarching goal is to obtain image quality consistent with the clinical objective, while avoiding unnecessary radiation. Too low an exposure, however, is also to be avoided if it results in an inadequate image. This Report represents an important continuation of NCRP reports on radiation safety and health protection in medicine and lays the foundation for the development and application of DRLs and achievable doses for diagnostic x-ray examinations. The concept of DRLs is extended to procedures other than diagnostic x-ray examinations (e.g., for interventional radiology) by the use of reference levels (RLs), which represent radiation dose levels that if exceeded prompt an evaluation of the reasons why. This Report discusses the establishment and use of RLs for fluoroscopically-guided interventional (FGI) procedures and describes why a different approach from DRLs is required to account for the greater complexity of interventional radiology compared with standard medical imaging procedures. Phantoms are models of the human body used in radiation dosimetry studies to estimate exposures to patients. The use of phantom survey data in the United States is contrasted with the use of patient-based dose data in Europe for establishing DRLs, achievable doses, and RLs. The use of phantom survey data is reviewed for determining DRLs for imaging modalities such as projection radiography, fluoro