Smoking in your car - the rules change today
However, it looks like one of those laws that could be very difficult to enforce, which is something the police are concerned about themselves.
Talking on Radio 4's Today programme, Steve White, chairman of the Police Federation, said: "The reality of the situation is we are struggling to attend burglaries. Should we be focusing on people smoking in cars with children in the cars or should we be focusing on burglaries?"
"The public need to decide what it is that they want their police service to do because we are at breaking point. We simply can’t be all things to all people."
Either way, the new law is basically telling you that you can't smoke in the car if someone is under 18 years old. At the moment, it only applies in England and Wales, but Scotland is looking at enforcing similar laws too. In Northern Ireland, they're going to see how everyone else gets on with the new law first, before doing anything.
A National Police Chiefs' Council spokeswoman said: "Police forces will be taking an educational, advisory and non-confrontational approach when enforcing the new legislation."
"This would see people being given warnings rather than being issued with fines, which would give time for public awareness of the offences to build."