Smokers can no longer light up in bars across Northern Ireland as the new smoking ban has come into force.
It is now illegal to smoke in workplaces, most enclosed public spaces and on public transport. The ban will be enforced by local councils.
Smoking is said to cause 3,000 deaths in NI each year, with one death every fortnight due to second-hand smoke.
The Republic of Ireland, Scotland and Wales already have smoking bans in place; England's starts on 1 July.
Smoke-free legislation is intended to protect public health by reducing exposure to second-hand smoke.
Health Minister Paul Goggins said the law would save lives, adding: "People do not want to breathe in others' smoke.
"Second-hand smoke is a toxic cocktail of around 4,000 chemicals - many of which cause cancer.
Where can you smoke?
In your home
In the open air
In a designated room in care homes, nursing homes and hospices
In a designated bedroom in a hotel, guest house, inn or hostel
Businesses will have to ensure no-one smokes in their premises and must display no-smoking signs. Councils have issued guidelines.
The new legislation has the support of major health charities and lobbying organisations across Northern Ireland.
These include the Health Promotion Agency, Ulster Cancer Foundation, Action Cancer, Macmillan Cancer Support, the British Medical Association, the Chartered Institute of Environmental Health, the Royal College of Nursing, Chest, Heart and Stroke NI and the Institute of Public Health.
Brian Gaffney, chair of the Smoke-Free Northern Ireland coalition and chief executive of the Health Promotion Agency, said: "This is one of the most important public health initiatives in Northern Ireland since the health service was established and it should be celebrated."
A new smoking area being created at Da Vinci's in Londonderry
Simon Clark, director of the smokers' lobby group Forest, said the ban made "Northern Ireland a less liberal, more intolerant place".
"Tobacco is a legal product and it is very sad that there is not a single pub, club or bar in the country where anyone can light up without being fined," he said.
Northern Ireland football star David Healy has given the ban his approval. He has urged parents to quit smoking altogether, rather than transferring their smoking from public places to their homes.
The Federation of the Retail Licensed Trade has been working with environmental health officers and Action Cancer and the Ulster Cancer Foundation in preparing publicans for the ban.
However, spokeswoman Nicola Carruthers said a backlog in planning applications meant that publicans were building smoking shelters without permission.
"With the planning system experiencing massive delays, of up to 18 months in some parts, many businesses are building beer gardens and smoking shelters without planning permission and applying retrospectively," she said.
"This is a major risk for many publicans, but they have to provide facilities for their customers who smoke or they may as well close their doors forever."
Smoke-free legislation was introduced in the Republic of Ireland from 24 March 2004, in Scotland from 26 March 2006 and in Wales from 2 April this year. It will also be introduced in England from 1 July.
Smoke-free legislation will be enforced by district councils' environmental health officers.
A telephone number has been set up to enable the public to report breaches of the legislation. It is 0845 603 2500.