The United States and Mexico have a special relationship under the North American Free Trade Agreement (NAFTA), which removes trade and investment barriers between the countries. The relationship has been strengthened by presidential visits, including President Clinton's February 1999 trip to Mexico and President Zedillo's June 2000 trip to the United States, and by regular contacts between cabinet-level officials. Major issues of concern to Congress are trade, immigration, drug trafficking, and political rights.