The age people can buy cigarettes in England, Scotland and Wales has gone up from 16 to 18.
It's the latest idea to try to stop people from lighting up, and the government hopes it will make it harder for kids to start.
Around one in 10 children aged 11 to 15 smokes, and the government hopes the change will bring that number down.
Studies have shown only a quarter of kids younger than 16 who tried to buy cigarettes or tobacco found it hard.
The government wants to stop young people from smoking
The change brings the smoking age in England, Scotland and Wales in line with other countries like Canada, Australia, New Zealand and America.
Earlier this year the government announced plans to put nasty pictures onto packs of cigarettes and tobacco, showing the effects smoking has on people.
Smoking in public places has already been banned in England, Scotland and Wales.