The tremor struck 15 km (10 miles) southwest of the capital Port-au-Prince, and was quickly followed by a series of strong aftershocks of up to 5.9 magnitude.
Thousands of homes, schools and hospitals were destroyed, as well as the U.N. headquarters in Port-au-Prince, the presidential palace and the main prison. Estimates of damage and losses range between $8 billion and $14 billion.
To make matters worse, a cholera epidemic started in October 2010 and spread across the country, killing thousands.
Four years after the quake, nearly 150,000 people are still living in tents and makeshift shelters in Port-au-Prince.
Slow reconstruction is compounded by donor fatigue, growing political instability and anti-government protests over high food prices and corruption.
INITIAL RESPONSE
The aid response was hampered from the start by the scale of the devastation. The quake destroyed much of the area's limited infrastructure - the main port was unable to function for several days, roads were destroyed, and there was a massive shortage of fuel to transport relief to the survivors.
Those best placed to organise a response - the government, United Nations and aid agencies based in Haiti - had lost staff, offices and computers in the quake.
Thousands of homeless fled to other parts of the country, but many settled in improvised camps around the capital which, in the first few weeks after the disaster, lacked food, water, decent shelter and medical care.
An estimated 4,000 people had to have limbs amputated, some because they did not receive medical attention in time.
Security was also a concern. The United States sent thousands of troops to help maintain law and order and deliver aid, and the United Nations boosted its peacekeeping force.
In January 2010, the United Nations launched a flash appeal for $575 million in emergency aid. The response was overwhelming, and contributions and pledges poured in. In February, the United Nations launched its largest ever humanitarian appeal for $1.44 billion.
Rescue, medical and relief workers from around the globe began pouring into Haiti hours after the earthquake struck.
Oxfam said the "unprecedented generosity" shown by the world for Haiti saved countless lives by providing water, sanitation, shelter, food aid, and other vital assistance to millions.
The U.N. World Food Programme helped close to 2 million Haitians with school meals, nutrition and cash-and-food-for work programs.
However, the aid operation was also strongly criticised for a lack of coordination, perhaps not surprising given that it brought together a struggling Haitian government, a plethora of U.N. agencies, other governments from around the world and a multitude of charities.
Wary of chronic government corruption, many nongovernmental organisations and aid donors set their own priorities, often with little coordination.
Of the thousands of charities working in Haiti, only a few hundred formally registered with the government and sent reports to the planning ministry as required by law.
The then Prime Minister Jean-Max Bellerive often complained that many charities in Haiti bypassed the government in their operations, some offering no reporting or accountability at all.
Before leaving Haiti in late 2010, the former Organization of American States representative Ricardo Seitenfus accused some humanitarian organisations of using the country as a "laboratory" and an opportunity to "do business".
The BBC reported that some people simply could not afford to leave the camps because the presence of foreign aid workers had pushed up rents.
In January 2011, Jean Renald Clerisme, who had one of Haiti's best paid jobs as senior adviser to then President Rene Preval, said he was being asked $2,000 to rent a house he wanted to move to - more than half his salary.
"When you have foreigners, the price goes up and the local cannot afford to pay," he told the BBC.
Aside from pushing up rents, Clerisme said foreign aid workers were actually encouraging more people to live in camps by concentrating nearly all their services around them.
Others said the influx of medical charities undermined Haiti's healthcare system because many medical staff quit to take better paid jobs with international agencies. The availability of free medical care meanwhile forced pharmacists out of business and doctors lost their patients.
An influx of free food similarly undercut local agriculture, according to Oxfam.
RECOVERY EFFORTS
The United Nations estimated that 70,000 buildings collapsed and tens of thousands were damaged, creating an estimated 10 million cubic metres of rubble – enough to fill 4,000 Olympic-sized swimming pools.
It took two years to clear about half the rubble. Much of it was done by hand on hillsides and densely populated areas of the capital that were inaccessible to heavy machinery.
In early 2012, aid agencies began moving from offering emergency aid to longer-term development projects. The aid operation involved about 12,000 aid groups. Then Prime Minister Gary Conille said in January 2012 that it was too scattered and lacked coordination.
Four years on, nearly 150,000 people are still homeless, most of them living in makeshift tent camps dotted around the capital in deteriorating conditions. Few of the camps have drinking water or enough toilets now that donors have pulled out.
Although the number of Haitians living in camps has fallen by nearly 90 percent from a July 2010 peak of 1.5 million, most people who have been relocated from camps have not moved into permanent housing.
Thousands of quake victims have been relocated after accepting cash and other assistance from aid groups, and many have been forcibly evicted by landowners. Many are now living in densely packed slums.
The capital had a major housing shortage even before the quake hit, with about 70 percent of residents living in slums.
In October 2013, the Haitian government launched the country’s first national housing policy in a bid to address the shortage of 500,000 new homes that it is estimated Haiti needs by 2020.
But Haiti Grassroots Watch, a local watchdog that monitors reconstruction, says government-led new housing projects are marred by corruption and mismanagement.
Poor coordination and lack of funds have also hampered rebuilding efforts, and disputes over land ownership have proved a major obstacle too.
Before building work can start, the government and aid agencies need to determine who owns what land - a major challenge after the earthquake killed some 16,000 civil servants and destroyed title deeds and land registry records.
Even before the quake, land ownership was a thorny issue in Haiti, contributing to violence and poverty in a country where land is concentrated in the hands of a few big landowners, known as grandons.
Few Haitians own land titles and there is no proper land registry system, with most titles passed down orally from one generation to the next.
The government says that resettling camp dwellers and building new homes and government buildings is a priority. The capital’s presidential palace and cathedral remain in ruins.
Haiti's recovery has also been affected by its first cholera outbreak in decades, which began in October 2010 in an area unaffected by the earthquake and quickly spread across the country. Since then, the water-borne disease has killed nearly 8,500 people and made nearly 690,000 ill. Haiti now has half the world’s suspected cholera cases.
In an attempt to stamp out the epidemic, donors have pledged $2.2 billion over the next 10 years to improve sanitation, medical infrastructure and access to drinking water.
An independent panel, appointed by U.N. Secretary-General Ban Ki-moon to study the epidemic, issued a 2011 report that did not determine conclusively how the cholera was introduced to Haiti.
However, last year, a Boston-based rights group, Institute for Justice and Democracy in Haiti, filed a lawsuit against the United Nations on behalf of cholera victims in Haiti. The lawsuit maintains the cholera epidemic was introduced by U.N. peacekeeping troops brought to Haiti from Nepal.
A disputed presidential election in 2011 and a political crisis that later deprived President Michel Martelly of a working government for months, also complicated the reconstruction efforts.
Many worry not enough is being done to provide Haitians with jobs and address deeply-rooted problems like education that could help Haiti pull itself out of crushing poverty and underdevelopment.
FUNDING RECONSTRUCTION
Damages and losses from the Haitian earthquake were evaluated at $7.9 billion, according to the World Bank, although estimates from other sources give higher figures.
At a major donors' conference in New York in March 2010, donors pledged $9 billion to support Haiti's five-year reconstruction and development plan.
An interim disaster management body was set up in April to decide which reconstruction projects to fund. It was co-chaired by former U.S. President Bill Clinton, the U.N. special envoy for Haiti, and by Prime Minister Bellerive. It later handed over to a government redevelopment authority.
The money is administered through the Haiti Reconstruction Fund, a multi-donor trust fund, supervised by the World Bank.
A major concern for donors is corruption which is rife in Haiti, particularly in the construction sector.
Most of the money donated between 2010 and 2012 bypassed Haitian institutions. Of the $6.04 billion in humanitarian and recovery funding less than 10 percent went directly to the Haitian government, according to Paul Farmer, U.N. special adviser on lessons from Haiti.
“Even the local NGOs and businesses were excluded: less than 0.6 percent of that $6.04 billion was invested in Haitian organisations and businesses,” Farmer wrote in Foreign Affairs. “One of the top bilateral donors in Haiti awarded only 1.4 percent of its contracts to local companies.”
U.S. companies and NGOs continue to receive the bulk of U.S. aid funding for projects in Haiti. The largest foreign aid donor to Haiti, the U.S. spent more than $270m on projects in Haiti in 2013, with U.S.-based companies receiving almost half of this and U.S. NGOs a further 37 percent, according to the U.S. Agency for International Development.
At a major donors' conference in New York in March 2010, donors pledged $9 billion to support Haiti's five-year reconstruction and development plan.
An interim disaster management body was set up in April to decide which reconstruction projects to fund. It was co-chaired by former U.S. President Bill Clinton, the U.N. special envoy for Haiti, and by Prime Minister Bellerive. It later handed over to a government redevelopment authority.
The money is administered through the Haiti Reconstruction Fund, a multi-donor trust fund, supervised by the World Bank.
A major concern for donors is corruption which is rife in Haiti, particularly in the construction sector.
Most of the money donated between 2010 and 2012 bypassed Haitian institutions. Of the $6.04 billion in humanitarian and recovery funding less than 10 percent went directly to the Haitian government, according to Paul Farmer, U.N. special adviser on lessons from Haiti.
“Even the local NGOs and businesses were excluded: less than 0.6 percent of that $6.04 billion was invested in Haitian organisations and businesses,” Farmer wrote in Foreign Affairs. “One of the top bilateral donors in Haiti awarded only 1.4 percent of its contracts to local companies.”
U.S. companies and NGOs continue to receive the bulk of U.S. aid funding for projects in Haiti. The largest foreign aid donor to Haiti, the U.S. spent more than $270m on projects in Haiti in 2013, with U.S.-based companies receiving almost half of this and U.S. NGOs a further 37 percent, according to the U.S. Agency for International Development.
THREAT OF MORE TREMORS
U.N. experts and aid agencies have urged the government and donors to commit to building earthquake- and hurricane-resistant schools, hospitals and houses. They say this would add less than 10 percent to building costs and save countless lives in the future.
Before the quake, Haiti's sprawling capital was littered with shoddy buildings due to poor construction standards and inadequate building regulations. Many homes were built on steep slopes and unstable foundations.
Training people to protect themselves during an earthquake can also save lives. Many in Haiti did not know the safest places to go, and some were killed in the aftershocks because they did not leave their buildings after the first quake struck.
Before the quake, Haiti's sprawling capital was littered with shoddy buildings due to poor construction standards and inadequate building regulations. Many homes were built on steep slopes and unstable foundations.
Training people to protect themselves during an earthquake can also save lives. Many in Haiti did not know the safest places to go, and some were killed in the aftershocks because they did not leave their buildings after the first quake struck.