This book, set out over three volumes, provides a comprehensive history of economic thought in the 20th century with special attention to the cultural and historical background in the development of theories, to the leading or the peripheral research communities and their interactions or controversies, and finally to an assessment and critical appreciation of economic theories throughout these times. It takes as its subject matter the canon of publications by major thinkers who self-consciously conceived of themselves as 'economists' in the modern academic sense of the term. It is a history of how, when and where the discipline of Economics took root in major universities and scientific communities of economists, and evaluates the emergence of different 'schools' of thoughts. Volume I addresses economic theory in the golden age of capitalism. It considers the contributions of Marshall, Pareto, Wicksteed, Schmoller, Bohm-Bawerk, Schumpeter, Wicksell, Fisher, Veblen and  other major thinkers, as well as the universities of Cambridge, Lausanne, Vienna, Berlin, and some others in US, before concluding with a look at the impact that the great war had on the discipline.  This work provides a significant and original contribution to the history of economic thought and gives insight to the thinking of some of the major international figures in economics as shown in major works published across the last 130 years. It will appeal to students, scholars and the more informed reader wishing to further their understanding of the history of the discipline.