Herbert Lionel Adolphus Hart was born in Yorkshire in 1907 to second generation Jewish immigrants. Having won a scholarship to Oxford University, he went on to become the most famous legal philosopher of the twentieth century. From 1932-40 H.L.A Hart practised as a barrister in London. He was pronounced physically unfit for military service in 1940, and was recruited by MI5, where he worked until 1945. During his time at the Bar he had continued to study philosophy and at M15 his interest was further stimulated by his philosopher colleagues in M16, Stuart Hampshire and Gilbert Ryle. After the war, Hart returned to Oxford to take up a philosophy fellowship, later to become Professor of Jurisprudence. H.L.A Hart single-handedly reinvented the philosophy of law and influenced the nation's thinking in the 1960s on abortion, the legalization of homosexuality, and on capital punishment. Hart's approach to legal philosophy was at once disarmingly simple and breathtakingly ambitious, combining as it did the insights of Austin and Bentham and the new linguistic philosophy of J.L. Austin and Ludwig Wittgenstein. He sought to elucidate a concept of law which would be of relevance to all forms of law, wherever or whenever they arose: his bestselling book, The Concept of Law, has sold tens of thousands of copies worldwide. In 1941, he married Jenifer Williams (a high-ranking civil servant, later an Oxford academic) with whom he had four children. Their relationship was an enduring if unconventional one. In the early 1950s, Jenifer was rumoured to be having a long-standing affair with Isaiah Berlin, one of Hart's closest friends. She was also, falsely, accused by the Sunday Times of having been a Russian spy, an allegation which was all the more scandalous given Hart's position at MI5 during the War. Nicola Lacey draws on Hart's previously unpublished diaries and letters to reveal a complex inner life. Outwardly successful, Hart was in fact tormented by doubts about his intellectual abilities, his sexual identity and his capacity to form close relationships. Her biography also sheds fascinating light on the origins of his ideas, and assesses his overall contribution. Above all, it chronicles of a life which had a depth ands impact far greater than many of Hart's readers have realized.