Like rabbits, guinea pigs have teeth that grow all the time, but thankfully they are usually worn down by all the gnawing and chewing they do. If you look at your guinea pigs front teeth (there are 2 at the top and 2 bottom) you may be surprised by how long they are. Each tooth may be 1-1.5cm long, but they should meet evenly, and then they will wear down evenly. However, sometimes a tooth may break, as the result of a fall, or from chewing on the cage wires, and if this happens the other teeth may need to be trimmed to make them all even again.
As the guinea pig gets older the back teeth may become overgrown, although this is not very common. Any guinea pig that stops eating should be taken to see a vet straight away.