This paper assesses the development of Niebuhr's thinking on the realist outlook in international relations and his attempt to link this as far as possible to ethical goals in world affairs. It will examine in particular Niebuhr's relevance to contemporary debate by focusing on Niebuhr's writings during and after the Second World War. The paper argues that it would be incorrect to perceive Niebuhr as simply a figure defined by the Cold War, for his writings contain a vision extending beyond issues of bipolar rivalry. It was this vision which led him to attack what he saw as the misuse of American power in South East Asia in the 1960s. Niebuhr should not be seen only as an apostle of naked realpolitik, important though this element is to his thinking. The earlier radicalism of the inter-war years left an important legacy of concern for social and economic justice that makes him a figure of continuing interest for scholars.