This book is a thorough description of the foundations of political economy created by Marx, its philosophical foundations, its objectives, its methodology, and the development of his ideas starting from the theory of the value. It discusses the role that Marx gives to capitalism in the full development of the productive potential of society, the eventual decline of this role and his predictions on the further development of the society, the nature of the crisis and his vision about the advent of socialism. At each stage of the analysis is a criticism is done based on what time has shown over the correctness of these predictions, and what the author considers are the achievements and the inconsistencies of the Marxist economic theory. It seeks to link the historical developments with the mentioned achievements and inconsistencies. Finally a discussion is carried on what must be discarded and what can be rescued of these theories as full of luminous ideas as duller results This book aims to rekindle the debate about a theory which has had important consequences on the past, present and likely the future of the world. Development of the themes is simple, and the author has sought to avoid tedious treatments. The work is accessible to a vast audience, not necessarily specialized in economics.