When the causes of the accident at Chernobyl Unit 4 on April 26, 1986, were studied, particular attention was given to the positive void reactivity coefficient and the dynamic characteristics of the shutdown system. The role of these factors in the development of the accident is discussed. The physical nature of the void reactivity coefficient is considered. Safety measures added to the remaining RBMK-type reactors are described. These measures include installation of 80 stationary neutron absorbers in the core to decrease the void reactivity coefficient as well as modification of the absorber rods. The results of reactor parameter measurements after these measures were implemented are presented. The calculation methods are outlined, and the changes in the neutron physics characteristics after the Chernobyl accident are described. The measures taken to improve the safety of RBMK reactors preclude the possibility of another accident of the Chernobyl type. Possible further improvements in the operation of an RBMK reactor are discussed.