A survey of fiber-optic sensors for the measurement of ionizing radiation is given. The interactions between ionizing radiation and fiber material, which are of significance for the fiber-optic detection of ionizing radiation, as well as the changes in the properties of the optical fibers induced by such interactions are discussed. Measurement and evaluation techniques for reading out the optical properties induced by ionizing radiation are described. A survey of possible applications for fiber-optic radiation sensors is given. The properties and application possibilities of fiber-optic dosimeters based on radiation-induced losses are specified. In particular, the requirements to be met by fiber-optic dosimeters for applications in radiation therapy are discussed. A fiber-optic dosimeter measuring system for radiation therapy based on radiation-induced losses is presented.