[en] The causes of the accident at Chernobyl Unit 4 on April 26, 1986 are discussed. The IAEA's International Nuclear Safety Advisory Group has reconsidered the root cause of the accident in the report INSAG-7. The shortcomings in the neutron physics characteristics of RBMK-type reactors were the positive void reactivity coefficient and the possibility of positive scram. The operating reactivity margin concept is seen as inappropriate for complete assurance that the control rod configuration secures fully adequate reactor protection. Two earlier accidents at RBMK reactors, a fuel channel failure at Leningrad 1 in 1975 and a fuel failure at Chernobyl 1 in 1982, had indicated weaknesses in the characteristics and operation of RBMK units. IAEA organized a consultants' meeting on void reactivity effects in RBMK reactors in Moscow 1995. In the meeting the phenomena influencing the void reactivity effects in RBMK reactors in Moscow 1995. In the meeting the phenomena influencing the void reactivity coefficient, the calculational tools and the status of experimental determination of the void reactivity coefficient in operating RBMKs were discussed. (orig.) (3 figs.)