This chapter discusses the role of diplomats at the Congress of Vienna, with a specific focus on the British Foreign Secretary, Robert Stewart, Viscount Castlereagh. While accepting that Castlereagh, like all other diplomats, was motivated by a realist maximization of Britain’s interests, for which the talk of peace and cooperation tosome extent constituted mere rhetoric, the chapter emphasises how diplomats like Castlereagh were formed by the experiences and viewpoints they had accumulated over a long period of time. It was this political socialisation, notably shaped by the lengthy struggle against French Jacobinism and Bonapartism, that influenced Castlereagh’s beliefs about the most dangerous threats to the security of the Continent, and by implication, of Great Britain. These mixed beliefs informed his decision-making in ways that promoted both British interests and collective security and peace in the years after 1813.