Marxists Internet Archive is a non-profit online encyclopedia that hosts a multilingual library of the works of communist, anarchist, and socialist writers, such as Karl Marx, Friedrich Engels, Vladimir Lenin, Leon Trotsky, Joseph Stalin, Mao Zedong, Rosa Luxemburg, Mikhail Bakunin, Peter Kropotkin and Pierre-Joseph Proudhon, as well as that of writers of related ideologies, and even unrelated ones. The collection is maintained by volunteers and is based on a collection of documents that were distributed by email and newsgroups, later collected into a single gopher site in 1993. It contains over 180,000 documents from over 850 authors in 80 languages. All material in the archive is provided free of charge to users, though not necessarily free of copyright.

Friedrich Engels was a German philosopher, political theorist, historian, journalist, and revolutionary socialist. He was also a businessman and Karl Marx's closest friend and collaborator.