I have a 65+ lb. lab mix and the Medium fit perfectly (her neck size is about 16.5-17 in.). It looks so comfortable because of the padding along the inside and I'm confident that she's secure because of the nickel-plated buckle. Also, I love the fact that I can hold the handle when we're going through traffic and busy areas, it's so convenient and such a great addition to the collar. 10/10 would highly recommend this for your pup!