A core argument of liberalism is that concentrations of unaccountable violent power are the fundamental threat to individual liberty and must be restrained. The primary means of restraining power are institutions and norms at both domestic and international level. At the international level institutions and organisations limit the power of states by fostering cooperation and providing a means for imposing costs on states that violate international agreements. Economic institutions are particularly effective at fostering cooperation because of the substantial benefits that can be derived from economic interdependence. Finally, liberal norms add a further limitation on the use of power by shaping our understanding of what types of behaviour are appropriate. Today, it is clear that liberalism is not a ‘utopian’ theory describing a dream world of peace and happiness as it was once accused of being. It provides a consistent rejoinder to realism, firmly rooted in evidence and a deep theoretical tradition.