While the common theory is that it's a sign your cat is happy as can be when they are in a purring mode.
According to Gudrun Ravetz, president of the British Veterinary Association that is not the case.
They are actually signalling for attention and trying to communicate with us.
"Dogs wag their tails and cats purr. Most of the time, it's because they are content and happy," Ravetz revealed.
Their purrs can range from happy ones to signs of feeling unwell or sad.
To distinguish the difference, Ravetz says “You should be able to guess how they are feeling by the situation they are in. If they are curled up on your knee, purring away, then it's safe to assume they are a content kitty”.