Firework displays, parties and other celebrations have taken place around the world to usher in the new year.
US revellers cheered the traditional ball drop in New York's Times Square to mark the start of 2011.
Earlier festivities ranged from the release of thousands of silver balloons in Tokyo, to the first countdown of a western new year in Hanoi, Vietnam.
In Sydney, crowds watched what is hailed as the world's biggest New Year's Eve fireworks display.
Dubai saw a spectacular laser and fireworks show at the Burj Khalifa, the world's tallest building.
Party-goers in Europe attended new year's events across the continent.
In London, crowds gathered on the banks of the Thames to watch fireworks. Some were launched from the London Eye big wheel - which is marking its 10th anniversary.
In Spain, crowds also gathered in Madrid's Puerta del Sol square to take part in "Las Uvas" (The Grapes), a tradition in which people eat a grape for each of the 12 chimes of midnight.
As the new year arrived, Estonia became the 17th country to join the euro currency, with Prime Minister Andrus Ansip marking the event by withdrawing euros from a cash point.
Meanwhile in Greece, troubled by an economic crisis, many people spent Thursday queuing to pay road tax on time or qualify for a tax amnesty.
In the Americas, some two million people gathered on Rio de Janeiro Copacabana beach beach for a fireworks and music show, and the country's logo for the 2016 Olympics was unveiled.
In the US, meanwhile, New York's municipal authorities and warmer weather combined to clear the streets following the snowstorm which blanketed the city this week, allowing hundreds of thousands to gather for the famous Times Square ball drop at midnight.
It was the tiny Pacific island nation of Kiribati that was the first to welcome in the new year at 1000 GMT.
Celebrations across Asia and Australasia followed, New Zealand - the first major country to see in 2011.
In Hong Kong, hundreds of thousands of people gathered along Victoria Harbour to watch fireworks explode from the roofs of the city's most prominent buildings.
At the Zojoji temple in central Tokyo, Japan, monks chanted as visitors packed in to count down until midnight. Thousands released a mass of silver balloons carrying notes with their hopes for the future.
And in Taipei, the capital of Taiwan, fireworks formed the shape of a dragon spiralling up the tallest skyscraper during the country's biggest-ever display - which cost $2m (£1.3m), and marked the beginning of year 100 on the Taiwan calendar.
In Sydney, about 1.5 million people came out with blankets and camping equipment ahead of the seven-tonne fireworks display above the Harbour Bridge.
Crowds began arriving more than 12 hours before the main display, with new visitors turned away as early as 1500 (0400 GMT).
This year marked the first time Vietnam's capital, Hanoi, officially celebrated a Western-style countdown. In previous years, the city authorities have focused on Tet, the holiday marking the lunar new year, which begins on 3 February.
In the Philippines, safety officials urged caution after firecrackers injured at least 245 people in recent days. According to tradition, many believe noisy celebrations drive away evil and misfortune.
In Burma, however, the military government has banned all fireworks and said severe action would be taken against anyone using them.
Political activist Aung San Suu Kyi, who was released from house arrest in November, called for the Burmese people "to struggle together with new strengths, new force and new words in the auspicious new year".
My girlfriend, Lucy, and I, have been travelling the world and are by Sydney Harbour Bridge for this New Year's Eve. We did London a couple of years ago but this new year should be so special - the atmosphere, the weather and already the fireworks (more going off as I type) will, I know, be something to remember! Tom, Sydney, Australia
Hello from a cold and snowy South Korea. I am spending the evening chilling with friends, cooking steak in red wine, drinking gin and tonics and Korean Soju. Philip Price, South Korea
I will spend New Year's Eve on my roof terrace in Jundrov overlooking the villages of Bystric and Komin. The forest starts about 100 metres behind us, which means in front we will have a 180-degree panoramic view of fireworks. This year I have bought some fireworks myself! Kim, Brno, Czech Republic
For us in Gulu, northern Uganda, for 24 years of conflict we had no time to celebrate new year in style but this time, we are preparing for a street bash and fireworks along Olia Road, in Gulu town, as we wait for the countdown to get us into the year 2011. Paero, Gulu, Uganda
My wife and I will celebrate with my parents-in-law and one or more friends, eating French and Swedish delicacies, drinking Champagne and different kinds of wine of course. The Swedish delicacies, because I'm Swedish and we'll prepare them ourselves, my wife and I. There are actually some things in the Swedish cuisine that the French appreciate but not much! Gunnar Bjursell, France
Twelve ex-pats from different countries around the world will be celebrating the new year with a South African style BBQ. Unfortunately without the booze, as we are on an alcohol-free oil and gas facility. John Hills, Kurdistan, Iraq
I'll just be on my balcony setting some fireworks off with a bottle of beer. Nice chilled way to start the year. Ruaridh Williams, Oberjosbach, Germany
A small group of us will gather beneath the bell tower at the First United Methodist Church and have a brief prayer, followed by ringing the bell at midnight. After some handshakes, hugs, and kisses we'll go home and go to bed. Dan Sinkhorn, Sullivan, Indiana, USA Spectacular firework show here. I've been living in Auckland for 17 years now and this is the best one I've seen out of all of them! Ryan Wong, Auckland, New Zealand
I prayed at Zojoji Buddhist Temple next to Tokyo Tower as thousands of balloons were released into the sky On the way home I went to a small, family run Zen Temple and was invited for zen meditation. Matt, Tokyo