Juan Carlos I has abdicated following a reign that spans five decades.
He is widely credited with helping smooth Spain's transition from Fascist dictatorship to modern democracy, having taken over the throne just two days after the death of strongman Francisco Franco.
Juan Carlos Alfonso Victor María de Borbón y Borbón was born in 1938 in Rome and spent his early years in Italy before moving to Spain in 1947.
He underwent courses at a number of military institutions, including the Naval Military School and the General Academy of the Air.
He married Princess Sofia of Greece in 1962 and they went on to have two daughters, Elena and Cristina, and a son, Felipe, who is now set to accede to the throne.
However, despite Spain being nominally a constitutional monarchy under Franco, there was no ruling monarch during the 39 years of his dictatorship.
That Juan Carlos was able to become King was made possible by a law signed in 1969 designating him as the next monarch.
This was only made possible by Juan Carlos agreeing to take the throne ahead of his father - also named Juan Carlos - who had vocally opposed the policies of the Franco regime.
Although he had pledged loyalty to Franco during the days of dictatorship, after he acceded to the throne the new King showed his support for democracy in 1976 by appointing the reformer Adolfo Suarez as Prime Minister.
The appointment was followed by the establishment of a modern parliamentary democracy, with a multi-party system, recognition for trade unions and guarantees of civil rights for Spanish citizens.
Although he was generally seen as a well-liked, respected figure for the majority of his reign, the latter years have seen a decline in both the royal family and Juan Carlos' personal popularity.
A corruption scandal involving the husband of Princess Cristina has tainted the image of the household. and an opinion poll published in January of this year found that two thirds of Spaniards wanted Juan Carlos to make way for Felipe.