Building on the work of John Rawls, this book offers a conception of ideal theory which provides practical guidance and a critical perspective on politics, institutions and society. The author develops this approach by discussing recent criticism of ideal theory by authors such as Amartya Sen and Raymond Geuss. Answering Sen's criticism, the author proposes a novel account of feasibility in relation to ideal theory, especially with regard to ideal institutional design. As a reply to Geuss' criticism, he discusses constructivist approaches of moral theory-building. Building on this discussion, the book develops an account of practical ideal-theoretical thinking that can be used for evaluating and criticizing societies and can guide institutional design under nonideal conditions.