Great collars. Tend to run pretty large though. I purchased 1 larges and 1 medium for my dogs (lab/collie and lab/pointer) and I have to say they ran quite big. Definitely adjustable, but I had to shrink down the medium and the large probably to a small and a medium just to fit the dogs. I am not sure what breed could use a XL because I am pretty sure a great Bernard could fit into the large. Regardless great collars. Just a little on the big size.