Since George Washington's oath of office in 1789, 45 presidents have taken part in the the solemn swearing-in ceremony of inauguration. The President of the United States holds the most powerful office in the world and must "take care that the laws be faithfully executed" as outlined by the Constitution way back in 1787.
As head of the Executive Branch of the government, the President of the United States acts as head of state and Commander-in-Chief of the armed forces. The Vice President is also part of the Executive Branch, ready to assume the Presidency should the need arise.