This chapter summarizes Levin-led investigations into tax haven banks facilitating tax evasion by U.S. clients. In one inquiry, whistleblowers expose how UBS opened secret Swiss accounts with $20 billion in assets for U.S. clients, and how Liechtenstein’s LGT Bank concealed its U.S. client accounts. Another inquiry exposes how Credit Suisse dragged its feet closing secret Swiss accounts for U.S. clients and was fined $2.6 billion. The chapter also examines U.S. enforcement actions taken against tax haven banks as well as the role of tax treaties. It also explains how the investigations helped spur enactment of landmark legislation known as FATCA to expose foreign bank accounts held by U.S. clients and helped inspire similar transparency measures abroad.