The first time I had to return this bc I bought a smaller size in store. My cat weighs about 10 lbs but is very petite. In thought no way is she a large. But the small was very tight on her, it didn't even buckle. I would say unless you literally have a kitten, buy the large (up to 40lbs of cat) and adjust the harness to fit them. Its much easier to put on the cat than other harnesses, my cat used to fight me on other types of harnesses but with this one she just slips right in.