Abstract: This article deals with certain problems of legal regulation of the relations from harm caused by high-risk sources in legislation of Russian and the United States. The authors rely not upon the examination of abstract models of functionality of the high-risk source as tool separated from the manufacturer, but rather the results of studying the legal norms (including legislative regulations and precedents), doctrine and judicial practice of both countries that provide answers to multiple argumentative questions. The scientific novelty consists in the fact that the authors consider the legal concepts and legislative solutions in the area of relations from the harm causes by high-risk sources (enhanced by hazardous activity) in the Russian Federation and the United States from the perspective of evolution of the scientific thought of these countries. The authors detect the common feature in both legislations – the doctrines of the Russian Federation and the United States contain the idea on the need for consideration not only the functionality of the tool (high-risk source), but also the actions of the operator of the high-risk source. Based on the conducted analysis of the physical essence of the high-risk source, is established the dialectical unity of the object, its properties and their possible transformation in the process of manufacturing or exploitation of the object. The analysis of physical essence with regards to resolution of the dichotomy of joint and several responsibility must be applied in future works dedicated to causing harm by the high-risk sources.