As of 2015, Presidents Andrew Johnson and Bill Clinton are the only two presidents of the United States that the U.S. Congress has impeached. However, Congress convicted neither president of the charges brought against him, and both were able to remain in office despite their impeachments.
On Feb. 24, 1868, the U.S. House of Representatives impeached Johnson for violating the Tenure of Office Act by removing Edwin Stanton as Secretary of War. The Tenure of Office Act required the president to receive Congressional approval before dismissing an appointed official. The U.S. Senate voted 35 to 19 in favor of finding Johnson guilty. However, this was one vote short of the required number of vote, thus acquitting Johnson.
In 1999, the House of Representatives impeached Clinton on two counts resulting from his conduct during the fallout from his affair with Monica Lewinsky. The Senate rejected both of the articles of impeachment sent over from the House.