Sedulius Scottus (fl. ca 850) is an important figure in the early history of European political thought, one of a group of ninth-century authors who produced short treatises in which they attempted to clarify the proper relation between spiritual and secular power. The Latin text of his De rectoribus Christianis (On Christian Rulers) is here presented in a critical edition more complete and accurate than anything hitherto available, with a facing-page English translation. The edition is supported by an Introduction setting it in the context of the general development of political theory in the Christian West. Dr R.W. Dyson was educated at the University of Durham, where he taught the History of Political Thought in the School of Government and International Affairs until his retirement in 2007.