Q&A: Right-to-die debate
The Supreme Court decision is the latest stage of a case that has been running since 2005. Tony Nicklinson, a man with locked-in syndrome, lost his court battle to legally end his life in 2012. Soon after, he became ill, and died a few days after the verdict. His widow, Jane, was joined in her application by another man - Paul Lamb, who put forward and lost the same argument in the court of appeal - and a third man known only as 'Martin'.
Why do these men want a change in the law?
Mr Lamb, who is 58 and from Leeds, was paralysed from the neck down after a car accident in 1990.
He says he endures pain every single day and does not want to keep living - but he has no way out. He is so badly paralysed that he could not take his own life, so he would need the help of a doctor to die.
'Martin', who has locked-in syndrome and cannot be identified for legal reasons, suffered a massive stroke in August 2008. He is unable to speak, virtually unable to move and describes his life as "undignified, distressing and intolerable". He wants to be allowed a "dignified suicide".
What are the different legal actions?
Mr Lamb is seeking a court declaration that any doctor who helped him to die would have a defence against a charge of murder. The defence is known as "necessity", meaning it was necessary for the doctor to act to stop intolerable suffering.
Mr Lamb won the right to join the litigation to continue the fight started by Mr Nicklinson. The British Humanist Association is also a party in the case.
And Jane Nicklinson is challenging the High Court's ruling in August that her husband could not end his life with a doctor's help.
Martin managed to persuade two of the three appeal court judges that the law on assisted suicide needs further clarification. The Director of Public Prosecutions (DPP) has to approve any assisted suicide court action in England, Wales and Northern Ireland.
In 2009 he issued guidance which made it clear that family or friends who travelled with a loved one to the Swiss suicide group Dignitas, would not risk prosecution.
As Martin's wife is unwilling to play any part in bringing about his death, Martin says he would have to ask a carer or nurse to help him - or a stranger - and that might involve payment.
Two of the three appeal court judges agreed that the DPP must clarify the law so that a health professional or anyone else would know if they risked prosecution for travelling with him to Dignitas.
Martin's case will now get a final ruling from the Supreme Court.
What does the current law say on this?
It is straightforward. If a doctor injected someone who wanted to die with a syringe full of lethal drugs in order to kill them, that would amount to murder.
Tony Nicklinson took his case for the right to die to the High Court where he asked judges to give a doctor a "common law defence of necessity" against any possible murder charge - but he lost his case in 2012.
He was paralysed from the neck down after a stroke in 2005 and suffered from locked-in syndrome. After refusing food and fluids, he died at home a few days after he lost his long court battle.
Legal experts say protecting a doctor who ends a single life, by giving him a defence for his actions, represents the most ambitious challenge yet to the law governing the right to die.
In Martin's case, current guidance makes it clear friends or family assisting a suicide out of compassion are unlikely to be prosecuted. But Martin's wife does not wish to be actively involved in his suicide - so he needs a doctor to end his life.
What is the current law on assisted suicide around the UK?
The 1961 Suicide Act makes it an offence to encourage or assist a suicide or a suicide attempt in England and Wales. Anyone doing so could face up to 14 years in prison.
But despite this, scores of UK citizens have travelled to Dignitas in Switzerland to end their lives - and no relative who has helped them has faced prosecution.
The law is almost identical in Northern Ireland.
There is no specific law on assisted suicide in Scotland, creating some uncertainty, although in theory someone could be prosecuted under homicide legislation.
Are there any guidelines on prosecution on assisted suicide?
After Debbie Purdy, who has severe multiple sclerosis, challenged the lack of clarity on the law on assisted suicide, the Director of Public Prosecutions set out the factors which would be taken into consideration when deciding if someone would face prosecution for helping someone else to die.
She wanted to know whether or not her husband would be prosecuted if he was to help her travel to a clinic in Switzerland to die. She won her case and guidance was issued in 2010 - but there was no change in the law.
The factors listed included whether the victim had reached a "voluntary, clear, settled and informed" decision.
There was also particular emphasis on the motivation of the suspect. They would be expected to have acted "wholly compassionately" and not for financial reasons, the DPP said.
However, the DPP said the individual circumstances of each case would still need to be investigated.
Is there any real chance the law could change?
There have already been several attempts to legalise assisted suicide, but these have been rejected.
The government has already said that there are no plans to change the law on assisted dying.
The Commission on Assisted Dying, established and funded by campaigners who have been calling for a change in the law, concluded in 2012 that there was a "strong case" for allowing assisted suicide for people who are terminally ill in England and Wales.
But many disagree. The medical profession and disability rights groups, among others, argue that the law should not be changed because it is there to protect the vulnerable in society.
The campaign group Care Not Killing says that doctors already have a lot of power and argues that it would be "dangerous to give them the authority to kill".
But in other countries, such as Belgium, Luxembourg and the Netherlands, legislation has been introduced to allow assisted dying. France and Spain are currently considering a reform of their laws.
Campaign group Dignity in Dying predicts that a lot more countries will follow suit.
The assisted dying legislation which it proposes would give terminally-ill, mentally competent adults choice and control over the timing and manner of their death.