The Gentle Leader headcollar has been scientifically designed to work with the dog"s natural instincts to control unwanted behaviour. The Gentle Leader enables owners to prevent their dog or puppy from pulling on the lead and makes training in basic obedience (sit, stay, down, come etc) really quick and easy because of its three-fold natural effect on the way dogs think, relax and behave. Gentle Leader simply & easily converts into a regular collar for greater flexibility. Small - Westie, Yorkie, Toy Poodle, Jack Russell Lhasa Apso etc Medium - Labrador, Coker/Springer Spaniel, German Shepard, Boxer, Retriever, Doberman, Staffie Large - Larger Labrador, Shepard breeds, Rottweiler, Great Dane, Newfoundland