This monograph highlights the versatile theoretical activity of Engels as one of the founders of Marxism, his contribution to the development of problems of dialectical and historical materialism, the philosophy of natural science and history, political economy and the theory of scientific communism. Various aspects of his work are considered in historical development. The book makes extensive use of Engels' literary legacy, including new publications of his works and letters, as well as achievements in the study of the history of Marxism. The book contains material to combat modern bourgeois and reformist critics and falsifiers of the teachings of Marx and Engels. It is designed for a wide range of readers interested in the history and theory of Marxism-Leninism.