Kalecki made important contributions to development economics, which rank him among the founding fathers of this area of our discipline. The objective of this paper is to give an account of his contributions, and in particular of his conception of the peculiarities and the way of functioning of the underdeveloped economies, and of the barriers that limits their capacity for high and sustained long run growth. As most socialist economists of his time, he was skeptic about the possibilities of overcoming underdevelopment under capitalism. However, in contradistinction to other pioneers of development economics, Kalecki did not stress the international forces that hamper development, but put the accent rather in the internal institutions and social and political determinants. In particular, the feudal and semi-feudal conditions in agriculture, the reduced market ensuing from income concentration and widespread monopolization of the economy, and the lack of willingness of entrepreneurs to carry out the necessary investments. Accordingly, his economic policy recommendations emphasized also the domestic aspects involved.