George Buchanan (1506-82) was one of the most distinguished humanists of the Northern European Renaissance. Hailed by his contemporaries as the greatest Latin poet of his age, he is chiefly remembered today as a radical political theorist whose Dialogus, first published in Edinburgh in 1579, justified the deposition of Mary, Queen of Scots in 1567 on the basis of a theory of popular sovereignty, which vested in the people the right to resist, depose and kill tyrannical monarchs. Immensely influential in radical circles both in Britain and on the Continent, it made a notable contribution to the debates over the nature and location of sovereignty which would finally bear fruit in the writings of John Locke. This critical edition and translation of the Dialogus makes available for the first time a modern scholarly version of one of the key texts in the history of early modern British political thought.