A chronology of key events.
1493 - Voyager Christopher Columbus claims Puerto Rico for Spain.
San Juan: An important port and tourist destination
Founded in 1508 by Juan Ponce de Leon
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.
Grito de Lares flag, a symbol of nationalist aspirations
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.
Rebel leader killed
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 August 2012 referendum to help decide island's political status.