In 2005 the WTO Appellate body ruled that the United States' total prohibition on cross border gambling services was unlawful under the General Agreement on Trade in Services (GATS). The questions raised by the case - whether and how a Government could block service provision on moralgrounds - went to the heart of key controversies surrounding international economic law. How do you reconcile a liberal system of international trade in services with national governments' desire to protect social values through service regulation? How much control are the WTO members willing totransfer to the WTO? How much regulatory diversity can the international trading system withstand?This book provides a comprehensive analysis of the regulation of services under the WTO's GATS Agreement. Through a thorough examination of the GATS negotiation history, substantive provisions, judicial interpretation, and ongoing reform process, the book presents a clear picture of how themultilateral trading system justifies and tolerates regulatory diversity. The book develops a horizontal framework for the assessment of whether a national barrier to trade in services is lawful, across all service sectors, focusing on the core general principles of necessity andtransparency.