Amelia Anne "Emily" Stewart, Marchioness of Londonderry (20 February 1772 – 12 February 1829), from 1794 until 1821 generally known as Lady Castlereagh (UK: /ˈkɑːsəlreɪ/), was the wife of the Georgian-era Irish statesman Robert Stewart, Viscount Castlereagh, who from 1812 to 1822 was British Foreign Secretary and Leader of the House of Commons. Well-connected by birth to the aristocracy and wife of a prominent politician who was Britain's leading diplomat during the close of the Napoleonic Wars, Lady Castlereagh was an influential member of Regency London's high society.