Professor Stephen Hawking has died at the age of 76, his family have confirmed.
His children, Lucy, Robert and Tim, said: "We are deeply saddened that our beloved father passed away today.
"He was a great scientist and an extraordinary man whose work and legacy will live on for many years."
They praised his "courage and persistence" and said his "brilliance and humour" inspired people across the world.
"He once said, 'It would not be much of a universe if it wasn't home to the people you love.' We will miss him forever," they said.
The former Lucasian professor of mathematics and theoretical physics at Cambridge, a post he held for 30 years, he was widely regarded as one of the most brilliant theoretical physicists since Einstein.
He was given only a few years to live after being diagnosed with motor neurone disease in 1963 at the age of 22.
However, he defied that prognosis for more than five decades, although he was paralysed for most of his life.
Professor Hawking has 12 honorary degrees, a CBE and was awarded the US Presidential Medal of Freedom in 2009.
''I'm sure my disability has a bearing on why I'm well known,'' he said in 2012.
''People are fascinated by the contrast between my very limited physical powers, and the vast nature of the universe I deal with.
''I'm the archetype of a disabled genius, or should I say a physically challenged genius, to be politically correct. At least I'm obviously physically challenged. Whether I'm a genius is more open to doubt."
Many experts said Professor Hawking defied medical science by reaching his 70s with such a debilitating condition.
New Zealand data scientist Edward Abraham studied theoretical physics under Professor Hawking at the University of Cambridge in 1990.
He said he was personable and approachable, and was very involved in the student life at the university.
"You know, he comes up to you in his wheelchair and you have to, like, initiate the conversation and kind of carry it, so that can be really intimidating.
"But he balanced that by being really encouraging, of all the students. He also used to be one of the staff who threw the student parties."
A life of science
Stephen William Hawking was born in Oxford on 8 January 1942. His father, a research biologist, had moved with his mother from London to escape German bombing.
Hawking grew up in London and St Albans and, after gaining a first-class degree in physics from Oxford, went on to Cambridge for postgraduate research in cosmology.
As a teenager he had enjoyed horse-riding and rowing but while at Cambridge he was diagnosed with a form of motor neurone disease which was to leave him almost completely paralysed.
As he was preparing to marry his first wife, Jane, in 1964 his doctors gave him no more than two or three years of life.
But the disease progressed more slowly than expected. The couple had three children, and in 1988 - although Hawking was by now only able to speak with a voice synthesiser following a tracheotomy - he had completed A Brief History of Time - a layman's guide to cosmology.
It sold more than 10 million copies, although its author was aware that it was dubbed "the most popular book never read".
Hawking discovered the phenomenon which became known as Hawking radiation, where black holes leak energy and fade to nothing. He was renowned for his extraordinary capacity to visualise scientific solutions without calculation or experiment.
But it was perhaps his "theory of everything", suggesting that the universe evolves according to well-defined laws, that attracted most attention.
"This complete set of laws can give us the answers to questions like how did the universe begin," he said. "Where is it going and will it have an end? If so, how will it end? If we find the answers to these questions, we really shall know the mind of God."
Hawking's celebrity status was acknowledged even by The Simpsons - he was depicted drinking at a bar with Homer, suggesting he might steal Homer's idea that the universe is shaped like a doughnut.
He also appeared as himself in an episode of the BBC comedy series Red Dwarf and as a hologram of his image in Star Trek: The Next Generation.
Undeterred by his condition, he continued his work as Lucasian Professor of Mathematics at Cambridge University, and in 2001, his second book - Universe in a Nutshell - was published.
He believed his illness brought some benefits; he said before he developed the disease he had been bored with life.
But his condition inevitably made him dependent on others. He often paid tribute to his wife, who had looked after him for more than 20 years, and friends and relatives were shocked when he left her for one of his nurses, whom he married in 1995.
By 2000, Hawking was a frequent visitor to the emergency department of Addenbrooke's Hospital in Cambridge, seeking treatment for a variety of injuries. Police questioned several people about allegations that he had been subjected to verbal and physical abuse over a period of years.
He was known to be an erratic, almost reckless driver of his electric wheelchair, and Hawking insisted his injuries were not caused by abuse. No action was taken.
In 2007, he became the first quadriplegic to experience weightlessness on board the so-called "vomit comet", a modified plane specially designed to simulate zero gravity. He said he did it to encourage interest in space travel.
"I believe that life on Earth is at an ever-increasing risk of being wiped out by a disaster such as sudden nuclear war, a genetically engineered virus, or other dangers. I think the human race has no future if it doesn't go into space. I therefore want to encourage public interest in space."
In 2014, the film The Theory of Everything was released, based on Jane Hawking's account of their courtship and marriage. Hawking himself met Eddie Redmayne as part of the actor's preparation for taking on the role of the scientist.
In a series for the Discovery Channel, he said it was perfectly rational to assume there was intelligent life elsewhere but warned that aliens might just raid earth of its resources and then move on.
He once wrote that he had motor neurone disease for practically all his adult life but said that it had not stopped him having an attractive family and being successful in his work.
"It shows," he said, "that one need not lose hope."
His passing has left an intellectual vacuum in his wake. But it's not empty. Think of it as a kind of vacuum energy permeating the fabric of spacetime that defies measure. Stephen Hawking, RIP 1942-2018. pic.twitter.com/nAanMySqkt— Neil deGrasse Tyson (@neiltyson) March 14, 2018
"Look up at the stars and not down at your feet. Try to make sense of what you see, and wonder about what makes the universe exist. Be curious." -- Stephen Hawking pic.twitter.com/Q3nmjWPVL0— Simran Jeet Singh (@SikhProf) March 14, 2018
Genius is so fine and rare. Goodbye Professor Hawking. You inspired and taught us all. pic.twitter.com/9Drdnv2eEe— Chris Hadfield (@Cmdr_Hadfield) March 14, 2018