Extra-Small fits puppies and toy breeds 10" through 16" chest.
Small fits dogs breeds similar to Miniature Poodle, Dachshuns, Schnauzers, etc. 12" through 20" chest.
Medium fits breeds similar to Terriers, Shelties, Cocker Spaniels, etc. 20" through 30" chest.
Large fits breeds similar to Shepherds, Retrievers, Dobermans, etc. 30" through 40" chest.