Changes introduced by the Lisbon Treaty, the European Union's (EU's) new reform treaty that took effect on December 1, 2009, have a significant impact on EU governance. The EU is an important partner or interlocutor of the United States in a large number of issues, but the complicated institutional dynamics of the EU can be difficult to navigate. The Lisbon Treaty makes substantial modifications in the leadership of the EU, especially with regard to the European Council, the Council of Ministers, and the EU's rotating presidency.