This review discusses radiopaques from research and development to application. Radiopaques attenuate X‐rays and may be either high atomic‐numbered materials or iodinated organic compounds. The latter may appear sometimes in liposomes or particulates. The theoretical and historical aspects, classification by structures, methods of synthesis, structure‐activity relationship, physical, chemical, and pharmacological properties, and related reactions, essential to the understanding of safety, tolerability, and efficacy of radiopaques in medical use, are included in the discussion.