This book considers the extent to which States are held accountable for breaches of jus cogens norms under international law. The concept of State accountability is distinguished from the doctrine of State responsibility and refers to an ad hoc practice in international relations that seeks to ensure States do not escape with impunity when they violate norms that are considered fundamental to the interests of the international community as a whole. State Accountability under International Lawsets forth a definition of State accountability and establishes a threshold against which the existence, or not, of State accountability can be determined. Using a Foucauldian influenced interpretive methodology, this book adopts a novel construction of State accountability as having legal, political and even moral characteristics. It argues that the international community seeks to hold States accountable utilising a variety of traditional and non-traditional responses that cumulatively recognise that the institutions that comprise and legitimise the State were instrumental in the particular breach. Using case studies taken from State practice from throughout the twentieth century and covering a range of geographic contexts, the conclusion is that there is evidence that State accountability, as it is conceptualised here, is evolving into a legal principle. The book draws together the many academic theories relating to accountability that have arisen in various areas of international law including environmental law, human rights and trade law before going on to examine an emerging practice of State accountability. A variety of ad hoc attempts and informal mechanisms are assessed against the threshold of State accountability established, with emphasis being given to practical examples ranging from the accountability of Germany and Japan after World War Two to the current attempts to seek accountability from Russia for former crimes of the USSR.