They often launch big campaigns around Christmas, warning people who are thinking of buying pets for presents about the responsibilities of looking after an animal.
Now it seems the problem is just as big in the summertime because loads of cats have kittens in the summer.
Often owners don't want to keep or find homes for the babies. And now so many people are abandoning their moggies, shelters are full to bursting point.
Animal charities are appealing for pet-lovers to give unwanted cats a new home to try to ease the problem.
The holiday season doesn't help, either.
Some people don't want to pay to put their pet in a cattery - a hotel for cats - while they are on their summer breaks.
Instead, they're dumping their cats. A group of 35 kittens was recently abandoned outside an animal shelter in Northern Ireland.