[en] Knowledge of the iodine inventory of the primary coolant and the release mechanisms from the fuel element during normal operation and transients is essential in HTR safety analysis. Under normal operating conditions the steady state release of the noble gases obeys a Booth type diffusion model. It has been shown that iodine release also follows this model. The experimental verification has been carried out by measurement of the iodine release of irradiation tests in MTR's and by comparison of the plate out experiments with the gas inventory in the AVR. Over a wide range an identical behaviour has been found except at low fractional releases. Here the iodine release is reduced significantly, presumably by adsorption. Measurements of gas release during temperature excursions in irradiation experiments indicated burst-like contributions not explicable by simple diffusion alone. It was supposed that similar bursts may dominate the transient release of iodine. First experimental results were obtained recently by on line detection of the iodine release from irradiation fuel in a special KFA hot cell facility. The released species were condensed on a cold plate that can be replaced during operation allowing monitoring of differential release. Several experiments used defective fuel in the range from normal operating temperatures to 1700 deg. C. Below 1300 deg. C the fractional release remained in the region of the recoil fraction from the kernel surface. Above this temperature there was a steep increase of the release up to 100% at about 1600 deg. C within 20 hours. The results clearly show non diffusional behaviour and were analysed with a phenomenological model including traps in the kernel material. A testing program has been carried out on iodine release from failed HTR-fuel. Release rates under both steady state and transient release were determined. The results of these investigations were evaluated with previously developed models. The quality of the fit is satisfactory. (author)