on Monday, bringing the curtain down on a reign of 39 years.
, when the country abolished the monarchy and replaced it with a democratic republic.
For much of his reign he was considered to be one of the most popular heads of state the world over. "
He went on to pay tribute to the outgoing sovereign as "a tireless defender of our interests," adding: "I am convinced this is the best moment for change".
The 21-year-old prince receives his officer's diploma after completing his training at the Military Academy of Zaragoza in 1959.
In May 1962 royal fans rejoiced as the future king married Princess Sofia of Greece and Denmark.
The ceremony took place in Athens at the church of Saint Dennis.
The newlyweds looked thrilled to be visiting President John F. Kennedy at the White House during their honeymoon in 1962.
A year after their wedding, the couple welcomed their first daughter Infanta Elena. Two years later their second daughter, Infanta Cristina, arrived. Then in 1968 the couple welcomed their son Felipe, Prince of Asturias, who will now take over the reins as Spain's King.
Juan Carlos speaks to the nation during the ceremony at which he was officially proclaimed king of Spain on 22 November 1975, two days after the death of General Franco.
Juan Carlos and Queen Sofia and their children were joined on holiday in Majorca by Prince Charles, Princess Diana and Princes William and Harry. Also shown in the picture, taken in 1989, are the king's children Elena, Cristina and Felipe.
The monarch has strong links to the British royal family. He is the great-great-grandson of Queen Victoria and is a distant cousin of Queen Elizabeth.
A proud moment for the football-loving king as he congratulates Spanish team captain Iker Casillas after the final whistle was blown during the Euro cup 2008 final. Spain had ended their 44-year wait for a major international title with a 1-0 victory over Germany.
A keen sailor, the monarch takes every chance he can to return to the open seas. He competed in the 1972 Summer Olympics in the dragon class event, finishing 15th.
The king became the most talked about attendee of the 2007 Ibo-American Summit when he told Venezuelan President Hugo Chavez off for interrupting Spain's president Jose Luis Zapatero. The phrase he used: "Por qué no te calles? (Why don't you shut up?)" became an instant sensation – spawning T-shirts, a domain name, mobile ringtones and appearing on TV programmes worldwide.
Shaking hands with President Mariano Rajoy, Juan Carlos officially hands over signed documents confirming his decision to abdicate on 2 June.