Existing knowledge of supplementary education, that is education organised and run by political, faith or ethnic groups outside of formal schooling, is patchy. This article is an exploration of the histories of supplementary education in the twentieth century. It is organised into three sections. The article begins by reviewing some existing literature and argues that supplementary education has been a topic of marginal concern for social historians, sociologists and historians of education. This marginal status has often been reflected in the way in which a dominant account of the history of supplementary education has entered the research literature despite a rather selective evidential base. The second section of the article deploys an expansive definition of education, and presents some new historical evidence concerning African Caribbean and Irish supplementary education. A final arguments section reflects on the significance of supplementary education and suggests some topics for a future research agenda. (Contains 83 footnotes.)