Subject and purpose of work: This paper presents origins and reasons underlying formulation of the theory of state interventionism in the free-market mechanism of economy. Broad view on classical economic theory from contemporary situation is the main aim of the work. Materials and methods: Author’s as well as other economists’ opinions have been employed to elucidate the essence and meaning of the concept of interventionism. A limited scope of information and the lack of dominance of assessment in shaping market equilibrium as well as the role of global demand and full employment have been indicated in this paper. Results: J.M. Keynes argued that a state should play an active role in regulating the economy and should be able to make quick intervention decisions. Beside the theory proposed by J.M. Keynes, this paper outlined views by P. Krugman, A. Marshall and other economists considered as Neo Keynesians. Conclusions: Comparison between liberal and intervention trends in economics leads to the concept of social market economy and contemporary studies on public economics.