This dissertation examines the transformations that affected the international economic system at the turn of the 1970s and 1980s by comparing the Keynesian and monetarist schools of thought. With regard to the industrialised countries, it also presents interpretations of the period’s economic problems associated with supply-side economics and, finally, with the Marxist school of thought. Excluding from the analysis those Second-World countries that had adopted the Soviet-style planned economy, the discussion follows an economic categorisation distinguishing between the long-standing capitalist and high-income industrialised areas of the world—such as North America, Western Europe, and Japan—the newly industrialised non-oil-exporting countries (NICs) in the Far East, such as South Korea, Taiwan, Thailand, Malaysia, Indonesia, as well as India, Pakistan, Morocco, Brazil, Argentina, and Mexico, which since the 1960s or 1970s had embarked, to varying degrees, on processes of mass industrialisation that would in the following decades lead to substantial increases in per-capita income, and finally the low-income countries located mainly in sub-Saharan Africa. The first part of the study justifies this classification on the basis of the growing divergence in economic conditions within the developing world, already evident during the 1970s and 1980s. The analysis then focuses on the main economic problems affecting each category in the same historical period. For the industrialised nations, these included the decline in productive investment, inflation, the failure to achieve full employment, the reduction of public investment, high private interest rates, and the strong interconnection among First-World economies, which rendered deflationary policies aimed at reducing imports while sustaining exports increasingly problematic. For the NICs, the key issue was the high level of external debt, while for the low-income countries it was the problem of debt default, brought about by a combination of three factors: balance-of-payments deficits and the resulting shortage of foreign reserves, weak credit ratings among international investors, and high public debt. The second part offers a diachronic account of the evolution of the international economic system from the 1950s and 1960s—the golden age of international Keynesianism—up to the 1980s, describing the dynamics through which the problems outlined in the first part, previously abstracted from their historical context, actually arose. Finally, the third part retraces the same historical period, explaining the developments that occurred in light of the consequences of the policies implemented, thereby providing an interpretation in terms of economic policy. A concrete case study—the application of these policies in Tanzania—is presented to highlight more specifically the consequences that the monetarist doctrine produced in low-income countries.