A chronology of key events

1493 - Voyager Christopher Columbus claims Puerto Rico for Spain.
San Juan

Now an important port and tourist destination
Founded in 1508 by Juan Ponce de Leon
Population: 433,000

1508 - Spanish settlement begins; Spanish explorer Juan Ponce de Leon arrives.

1898 - Under the Treaty of Paris, Puerto Rico is ceded by the Spanish to the US at the end of the Spanish-American War.

1900 - US Congress institutes civil government in Puerto Rico under the Foraker Act. US maintains strict control over island affairs, prompting demands for more local control.

1917 - Jones Act grants US citizenship to Puerto Ricans.

1946 - US appoints Jesus T Pinero as Puerto Rico's first governor.

1947 - Partial self-government granted, enabling Puerto Ricans to elect their own governor.

1948 - Munoz Marin elected as governor by popular vote. As leader of the Popular Democratic Party (PPD), he serves four terms.

1950 - October - President Truman signs the Puerto Rico Commonwealth Bill, paving the way for a Puerto Rican constitution. Nationalists oppose the new law and resort to violence. In November two US-based nationalists attempt to assassinate President Truman in Washington DC.

1951 - Puerto Ricans vote overwhelmingly in favour of US commonwealth status in a referendum.

1952 - 25 July - Puerto Rico's constitution is proclaimed, establishing a commonwealth with autonomy in internal affairs.

1954 - Nationalists open fire in the US House of Representatives, injuring five congressmen.

1963 - World's largest radio telescope inaugurated at Arecibo Observatory.

1967 - Referendum on island's political status: 60% of voters back continued commonwealth status.

1985 - Landslide in Ponce kills 129 people.

1993 - Spanish and English declared as official languages.

Nearly 49% of voters back continued US commonwealth status.

1998 September - Hurricane George sweeps across the territory causing serious damage. President Clinton declares Puerto Rico a disaster zone.

1999 April - Off-target bombs kill a security guard on Vieques, an island which is home to a US Navy bombing range, triggering large-scale protests.

2000 November - PPD's Sila Calderon elected as first female governor.

2001 June - President George W Bush agrees to halt the use of the Vieques bombing range by May 2003.

2003 1 May - US Navy closes its bombing range on Vieques.

2004 December - Ruling party's Anibal Acevedo Vila is confirmed as the territory's next governor after nearly two months of legal wrangling over a 0.2% margin of victory.

2005 July - Voters in a referendum back the idea of replacing the island's Senate and House of Representatives with a one-house legislature.

2005 September - Fugitive pro-independence rebel leader Filiberto Ojeda Rios is killed in a shoot-out with US federal agents. The handling of the incident causes anger on the island.

2006 March - US Supreme Court rejects an appeal calling for Puerto Ricans to have the right to vote in US presidential elections.

2006 May - Budget shortfall leads to the closure of schools and many government agencies. Thousands of people take to the streets in protest.

2006 November - Puerto Rico adopts its first sales tax, aiming to address major budget deficits.

2008 March - Governor Acevedo Vila pleads not guilty to corruption charges in connection with alleged irregularities over the funding of election campaigns between 2000 and 2004.

2008 June - Mr Acevedo Vila calls on the UN to back Puerto Rico's right to self-determination.

2008 November - Luis Fortuno of the pro-statehood New Progressive Party wins gubernatorial election and is inaugurated in January.

2009 May - tens of thousands of workers march through San Juan in protest at possible job layoffs after Governor Fortuno announced a major programme of public spending cuts in March.

Former governor Anibal Acevedo Vila found not guilty on all counts in a corruption trial linked to allegations of raising and hiding thousands of dollars in illegal election campaign contributions.

2009 October - Whole neighbourhood near San Juan evacuated after an explosion at an oil refinery triggered a major fire. The blast was measured to a strength of a 2.8-magnitude earthquake.

2010 October - Dozens of Puerto Rican police are held on drugs charges following the biggest police graft probe in FBI history.

2011 June - President Barack Obama becomes first president since John F. Kennedy to make an official visit to Puerto Rico.

2011 December - Governor Fortuno gives go-ahead for 2012 referendum to help decide island's political status.

2012 August - Voters in referendum reject proposed constitutional changes that would have reduced the size of the territory's legislature and given judges the right to deny bail in certain murder cases.

2012 November - Voters back a non-binding referendum to become a full US state. The measure requires US Congressional approval, but President Obama says he will respect the vote. US Congress begins to consider at least two bills on the status of Puerto Rico.

Popular Democratic Party candidate Alejandro Garcia Padilla beats incumbent Governor Luis Fortuno in a very close gubernatorial race. He is sworn in as governor in January 2013.

2013 April - President Obama proposes another, federally-funded referendum on the status of Puerto Rico in order to break the Congressional impasse on the issue.

2013 October - Puerto Rico faces a mounting debt crisis, raising the possibility that it might require federal assistance.

2014 July - Governor Alejandro Padilla pledges to hold another plebiscite on Puerto Rico's status by 2016.

2015 June - Governor Padilla warns that Puerto Rico cannot pay its $72bn debt, and is close to defaulting on a large round of payments. Ultimately, the island narrowly avoids default.

2015 August - Puerto Rico defaults on its large debt for the first time, missing payments worth more than $50m.

2015 October - President Obama urges Congress to devise a plan for Puerto Rico's massive debt in order to avoid a "humanitarian crisis".

2015 December - Puerto Rico narrowly avoids default on a debt payment, as the government said it was struggling to find money for government services and future debt payments.

Governor Padilla says he will not seek re-election, and would instead focus his efforts on the debt crisis.

2016 January - Puerto Rico defaults for a second time, as the economy continues to struggle. Around $37m was left unpaid. Governor Padilla continues his push for the island to be granted greater bankruptcy protections by the US.

2017 May - The US territory declares bankruptcy - the largest ever by a local US government - after being unable to pay its debts.

2017 June - Puerto Rico votes overwhelmingly in favour of becoming America's 51st state in a non-binding referendum, though only 23 percent of voters take part.

2017 September - Two hurricanes leave a trail of devastation and generate a political row over allegedly tardy US relief efforts.