Kalecki's opus has been acknowledged chiefly as a contribution to the theory of distribution and the business cycle. Little attention has been given to the theory of effective demand and to unemployment equilibrium, i.e. to the field traditionally covered by Keynesian economics. This book is an attempt to draw attention to the most innovative core of Kalecki's thought on capitalist economies, which is also strictly interrelated to the history of economic thought. Accordingly, it focuses on the relationships with other theoretical approaches, to methodology and the theory of effective demand and investment, to the theory of distribution and prices, and to the theory of money.