T he accident at the No. 4 reactor of the Chernobyl Power Station took place on April 26,1986, at 1:23 AM. Like the accident at Three Mile Island No. 2, the immediate cause was operator error, but the much more serious release consequences in the Ukraine can be ascribed to reactor design [l-3]. Two explosions occurred, the first due to steam and a second one due to hydrogen. The explosions expelled fission products and fuel elements to the exterior that accumulated in a cloud reaching to approximately 7,000 m and centered at approximately 4,500 m. Because the graphite ignited, there was a second, more prolonged but less intense, release over a 9to lo-day period that peaked on May 6,1986, and dropped sharply on May 11 as the fire was extinguished. The following volatile elements, as well as the noble gases xenon and krypton, constituted the most abundant released material: iodine, cesium, and tellurium. In this review, we will deal primarily with iodine isotopes as they affected Poland. Although iodine isotopes other than i311 are present with higher core inventories [2,4], their short half-lives make them important primarily in the first days, and near the reactor. Spectroscopy of the environmental isotopes was not possible under the emergency conditions prevailing, but it was found that approximately 28% of the air radioactivity was present as short-lived isotopes of iodine as measured in Warsaw on April 28. Thus, some inhalation of short-lived iodine isotopes occurred during the earlier exposure period. Nevertheless, the timing was such that the protective actions were directed at lslI. Initial Russian estimates of the amount of i3iI released from a core inventory of approximately 63 MCi were 7.3 MCi [5]. More recent measurements and calculations have raised that estimate to approximately 60% of the inventory and total releases of 36 to 46 MCi i3iI (1.3 to 1.7 ExaBq) [2,4,6,7]. In Poland, increased air radioactivity was first detected on the night of April 27 and 28 [7,8] and amounted to 504 Bq of radioiodines/m3 air with 1.55to 3.0~times higher values in northeast Poland. By 10 AM on the 28th, all Polish monitoring stations reported air, soil, and wa-