Daniel De Leon, alternatively spelt Daniel de León, was a Curaçaoan-American socialist newspaper editor, politician, Marxist theoretician, and trade union organizer. He is regarded as the forefather of the idea of revolutionary industrial unionism and was the leading figure in the Socialist Labor Party of America from 1890 until the time of his death. De Leon was a co-founder of the Industrial Workers of the World and much of his ideas and philosophy contributed to the creations of Socialist Labor parties across the world, including: Australia, the United Kingdom, Canada, and the Socialist Trade and Labor Alliance.

In Marxism, a theoretician is an individual who observes and writes about the condition or dynamics of society, history, or economics, making use of the main principles of Marxian socialism in the analysis.