[en] Since the first years of development of nuclear power the most serious attention has been given to the planning of measures of population protection in the event of a radioactive release to atmosphere from a nuclear reactor. In the 60s 'Criteria for urgent decision making in the event of an accidental radioactive release into the environment' were developed in the USSR. When substantiating numerical values of potential radiation doses reasoning the implementation of countermeasures, specific conditions of emergency situations, characteristics of countermeasures and the real possibilities of timely dosimetric estimation of the situation were considered. The 'Criteria' were designed for urgent decision making at an early stage, in the first hours and days following the emergency. After the start of the Chernobyl accident on April 26, 1986, decisions on measures of protection of the population living in proximity to the site of the accident, including relocation of residents of the town of Pripyat on May 27, 1986, were taken on the basis of this document, as well as decisions for iodine prophylaxis and for relocation of other settlements within the 30 km zone. The decisions were taken by the result of the estimation prediction of the radiation situation which showed a possibility of an excess of criteria levels by external gamma radiation and by inhalation of radioiodine