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Important results obtained from investigations of the diverse processes that occurred during the accident in the No. 4 unit of the Chernobyl nuclear power plant on April 26, 1986 are presented in a concise form. The events occurring in the unit in connection with the preparation for runout tests to be performed on a turbogenerator are examined: from the characteristics of the test program to the actions taken by the plant personnel in preparation for the these tests. The events that determined the character of the evolution and the catastrophic consequences of the accident are noted: involuntary stopping of the reactor and subsequent power increase without unpoisoning of the core, stopping of feed-water flow with increasing loop flow, operators' lack of information about the actuation of the technological shielding of the electric drives of the main circulation pumps and electricity generator. Computational analysis of the initial phase of the accident and other computational assessments revealed new factors which with a large void effect made reactor runaway on prompt neutrons unavoidable -cavitation steam reaching the core entrance from the shutoff-regulation valve and/or the cavitation shutdown of the feed to the remaining main circulation pumps after the pumps powered by the run-out turbogenerator were disconnected. The most likely picture of the evolution of the accident is constructed on the basis of an examination of the destroyed power-generating unit.The accident which occurred in the reactor in the No. 4 unit of the Chernobyl nuclear power plant on April 26, 1986 is the largest technogenic catastrophe of the 20th century. This extremely large reactivity accident at a nuclear reactor had serious economic, sociopolitical, and humanitarian consequences. This is why the main efforts made during the entire post-accident period were geared toward studying and liquidating its consequences. The investigations, in which domestic and foreign specialists participated, of the accident processes themselves were supposed to determine, first and foremost, the reasons for what happened in order to prevent such a development of events to be repeated at operating RBMK reactors. It was concluded that the accident occurred because the neutron-physical characteristics of the core were unsatisfactory and the emergency shielding was actuated too slowly; these factors appeared as a result of incorrect actions taken during the preparation for and conduct of runout tests on a turbogenerator with only the internal plant load. Understanding these reasons made it possible to develop technical and organizational measures to increase the safety of operating nuclear power plants with RBMK reactors. Adopting these measures in stages made it possible to decrease the void effect of reactivity to acceptably low values in all RBMK reactors and substantially increase the actuation speed of the emergency shielding.