We bought these about 5 yrs ago for our 2 Australian Shepards who love to pull. I am back to buy them again as 1 no-pull trainer finally broke on the slideable plastic piece that rests on their chest. They greatly reduce the pulling power of our 45 lb dogs. A question I had when trying to purchase through Amazon was what size to get. Could not find the breakdown of Sm, Med, Large anywhere including Four Paws own Web site. I guessed Med & was right. Here is the sizing: Sm = Jack Russels, pugs etc. Med = Cockers Spaniels, Bulldogs etc. Large = Great Danes, Rottweilers etc.