When Barack Obama praised the writings of philosopher theologian Reinhold Niebuhr in the run up to the 2008 US Presidential Elections, he joined a long line of top politicians who closely engaged with Niebuhr's ideas, including Tony Benn, Jimmy Carter, Martin Luther King Jr. and Dennis Healey. Beginning with his early ministry amongst industrial workers in early twentieth century Detroit, Niebuhr displayed a passionate commitment to social justice that infused his life's work. Rigorously championing 'Christian Realism' he sought a practically orientated intellectual engagement with the political challenges of his day. His ideas on International Relations have also helped to shape debate amongst leading academic thinkers and policy makers. In both Christian and secular contexts he continues to attract new readers today. In this timely re-evaluation both critics and disciples of Niebuhr's work reflect on his notable contribution to Christian social ethics, the Christian doctrine of humanity, and the engagement of Christian thought with contemporary politics. The authors bring a wide range of expertise from both sides of the Atlantic, indicating how a re-evaluation of Niebuhr's thought can help inform contemporary debates on Christian social ethics and other wider theological issues.