A chronology of key events
1493 - Voyager Christopher Columbus claims Puerto Rico for Spain.
1898 - Under the Treaty of Paris, Puerto Rico is ceded by the Spanish to the US at the end of the Spanish-American War.
1917 - Jones Act grants US citizenship to Puerto Ricans.
1947 - Partial self-government granted, enabling Puerto Ricans to elect their own governor.
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.
1993 - Spanish and English declared as official languages.
2006 March - US Supreme Court rejects an appeal calling for Puerto Ricans to have the right to vote in US presidential elections.
2006 November - Puerto Rico adopts its first sales tax, aiming to address major budget deficits.
2012 November - Voters back a non-binding referendum to become a full US state. The measure requires US Congressional approval, but President Barack Obama says he will respect the vote. US Congress begins to consider at least two bills on the status of Puerto Rico.
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.
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.
2019 July - Governor Ricardo Rosselló resigns after days of street protests over a group text message scandal involving offensives comments.