Impeachment in the United States is an expressed power of the legislature that allows for formal charges against a civil officer of government for crimes committed in office. The actual trial on those charges, and subsequent removal of an official on conviction on those charges, is separate from the act of impeachment itself. Impeachment proceedings have been initiated against the President of the United States on several occasions. Andrew Johnson and Bill Clinton are the only two presidents to have been successfully impeached by the House of Representatives, though both were later acquitted by the Senate. The impeachment process of Richard Nixon was technically unsuccessful, as Nixon resigned from office prior to the full-House vote for impeachment. To date, no U.S. President has been removed from office via impeachment and conviction.