We report on our irradiation experiments on different types of fiber-optic sensors, including three types of commercially available temperature sensors, a multimode extrinsic Fabry-Perot cavity strain sensor and fiber Bragg- gratings. For the temperature sensors, results show that gamma radiation does not interfere with the basic sensing mechanism and that the most critical component turns out to be the optical fiber itself. Semiconductor absorption temperature sensor showed no degradation up to total doses of 250 kGy, whereas the specifications of Fabry-Perot type sensor and fluorescence temperature sensors were already dramatically influenced below the kGy-level. Replacing the optical fiber by a more radiation resistant version allowed to increase the radiation hardness of the fluorescence sensors system by orders of magnitude. The use of fiber- optic sensors in the presence of neutron radiation remains compromised. Similar conclusions are valid for the Fabry- Perot type fiber-optic strain sensors. We finally show that the Bragg-grating resonance wavelength can shift with radiation dose, but that the temperature sensitivity remains unaltered.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.