This paper inquires into the interaction paths which exist between economic knowledge and public policy, and it proposes a framework of analysis in which three types of factor intervene, namely those relative to the generation and diffusion of knowledge itself, those relative to the evolution of economic performance and those relative to institutional determinants. This framework is applled to the analysis of the process of change -unfinished, uneven and contradictory- which operated in the 1930's, both in the theoretical and in the practical economic policy of the developed world. Two principal ideas are brought out in the conclusions: first, the Keynesian theoretical revolution, with its aura of inte-llectual rigour, legitimised the growing inclination of economists to experiment with a greater role and intervention on the part of public bodies and, secondly, the institutional conditions of the economic political process (political, social, administrative) played a key role in the form and pace of incorporation of new ideas to these processes.

Keywords: history of economic thought, emergence of economic policy.