In the century following his execution for treason in 1683, Algernon Sidney became one of the most widely influential political writers - in both Europe and America - that England had ever produced. This is the first full-scale study of Sidney for more than a century, and the first ever study of his political thought. The book describes Sidney's republican political ideas and their later impact. It sets them in their ideological context, in relation both to their sources and to the ideas of contemporaries, including Milton, Harrington, Vane, and Locke. It then asks: how did this ideology develop, and why? The answer involves a series of investigations: of Sidney's family background; of the nature of his personal life and family relationships; and of his public political career. On this latter score we follow Sidney's progress from parliamentarian soldier in the English Civil War, to senior member and ambassador of the English Republic, to embittered exile after the Restoration in 1660.