Abstract This article reviews the principles of protection from ionizing radiation in diagnostic imaging, with emphasis on justification, optimization, and quality assurance. It explains the importance of balancing diagnostic benefit with the reduction of unnecessary exposure and discusses the protection of patients, workers, and the public, alongside the medical physicist’s role in radiation-safety programs, protocol review, and diagnostic reference levels. Keywords Radiation protection; justification; optimization; ALARA; dose; quality assurance; diagnostic imaging. Introduction The use of ionizing radiation in medicine is an important application of physics in diagnosis. However, its use must be justified and optimized. The World Health Organization emphasizes that safe use requires maximizing benefits and minimizing harm through justification, optimization, and the prevention of unintended exposures. The Principle of Justification Justification means that a radiological examination should not be performed simply because equipment is available. The expected diagnostic benefit must outweigh the potential harm associated with radiation exposure. This principle is the first foundation of radiation protection. Optimization and the ALARA Principle Once an examination is justified, protection should be optimized so that the dose is as low as reasonably achievable, while maintaining image quality appropriate to the diagnostic task. This is known as the ALARA principle. This does not mean using the lowest possible dose in absolute terms, because excessive dose reduction may degrade image quality and increase the likelihood of diagnostic error. Patient Protection Patient protection includes selecting an appropriate protocol, accurately limiting the imaging field, avoiding unnecessary repeat examinations, reviewing exposure parameters, and considering age, body size, and clinical condition. These measures are especially important for groups that are more sensitive to radiation. Protection of Workers and the Public Occupational protection is based on minimizing exposure time, maximizing distance from the radiation source, using appropriate shielding, and following operating procedures. Unauthorized entry into exposure areas should also be prevented, and work should be organized to reduce unnecessary exposure. Quality Assurance as a Radiation-Safety Tool Equipment malfunction or poor calibration may increase radiation dose or reduce image quality. Quality-assurance programs are therefore directly linked to radiation protection: stable equipment performance helps ensure predictable results at an appropriate dose. The Role of the Medical Physicist Medical physicists contribute to dose assessment, equipment performance testing, imaging protocol review, investigation of unintended exposures, staff training, and implementation of international standards. Conclusion Radiation protection is part of an integrated system that includes diagnostic image quality. Combining justification, optimization, and quality assurance is fundamental to safe and effective medical imaging practice. Almustaqbal University – The First University in Iraq