Radioiodines have a long history in nuclear medicine. Herein, we discuss the production, properties and applications of these versatile iodine‐based imaging and theragnostic agents. There are 38 isotopes of iodine (I) including one stable form (127I). The most common radionuclides used in medical imaging and treatment, including Iodine‐123 (123I), Iodine‐124 (124I), Iodine‐125 (125I) and Iodine‐131 (131I), are discussed in this review.