The relevance of the study of economic inequality as the most important problems of political economy is emphasized, in economic, sociological and political aspects. The author discusses the hypothesis by Simon Kuznets who won the Nobel Prize in Economic Sciences for the impact of economic growth on the non-uniformity of distribution, based on an analysis of historical dynamics of income inequality. The article summarizes the methodological approaches to the study of the historical dynamics of inequality in income distribution, relationship between economic inequality and economic growth. It is shown that in the middle of the XX century, the concept of the natural cycle of inequality was dominated. However, there were not consensus in more recent studies about whether economic growth can be accompanied by a decrease, increase or preservation of inequality, while the growth of inequality may occur against a background of economic downturn.Special attention is paid to the comparison of the liberal economic approach, according to which inequality is a key tool for economic growth: the greater the inequality, the greater the economic growth - and the opposite point of view, which other researchers follow, viewing the inequality as a constraint for economic growth. The urgency of the problem of inequality in the Soviet period and its study is shown. The researchers of that time faced with the problem of differences between the real life and officially provided data. A significant contribution to the development of methodological base for political economy studies of inequality was made by J.A. Kronrod and A.P. Butenko. Their results served as the impetus for reformatory ideas about the economic theory of socialism, which are of importance today as well.Empirical data showing inequality trends in the Soviet and post-Soviet periods are provided. It is shown that the market reforms in post-Soviet Russia has resulted in excessive income differentiation of the population and, as a consequence - sharp social polarization of society. It is concluded that at the present time in Russia the issue at stake is not income inequality as such, but rather its transformation into particularly dangerous form of social and economic inequality. Unjustified excessive inequality of incomes among the Russians is one of the major factors constraining economic growth, which is closely related to the value of consumer demand.