The first volume of the correspondence of Marx and Engels, which is connected to the writing and publication of "Capital," includes letters containing original formulations of important theoretical positions and reveals the constant intellectual communication and collaboration of the two classics of the theory of scientific communism. This publishing initiative is the product of the decades-long effort of the Modern Era to translate and publish an increasingly large part of the rich and diverse legacy of Marx and Engels in Greek. The letters or excerpts of letters included in the edition shed light on important aspects of "Capital" and the debates surrounding it, while highlighting the development of the thoughts of Marx and Engels during the writing and editing of the material in the three volumes of "Capital." At the same time, they bring to the forefront a less illuminated but extremely significant aspect of the challenge of publishing "Capital," which is none other than the extremely difficult conditions under which it was written and published. Within the texts of the edition, answers can be found to questions such as: How many and which plans did Marx change for the publication of "Capital"? What was Engels' role in the publication of the three volumes of "Capital"? What role did the very large economic and family difficulties, as well as the health problems faced by Marx, play? What is the relationship between the philosophical elaboration of Marx and Engels and the publication of "Capital"? What is the relationship between the Critique of Political Economy and "Capital"? On what occasions were various pamphlets written, such as "Wages, Price, and Profit"?