Normally sheep live for 11 or 12 years, so now scientists are trying to work out if Dolly died earlier than that because she was a clone.
Dolly first fell ill in January 2002 with arthritis, a bone disease that normally affects animals much older than Dolly was.
Is cloning right or wrong?
Some people think cloning animals is wrong, and may say that her death at an earlier age than expected is proof that cloning is wrong.
But the scientists who created her say that cloning animals could help people battle diseases in the future.
Find out more about animal cloning and why it happens in our guide - the link is at the top of the page.