All of our dog harnesses are adjustable in the chest as well as around the body for maximum comfort and adjustability. There is a side-release buckle around the body for convenience.  The small dog harnesses are made of 3/8" wide nylon and adjusts from 9"-14" around the body. The medium dog harnesses are available in two different widths. The 5/8" wide nylon adjusts from 13"-20" around the body and the 3/4" wide nylon adjusts from 15"-25" around the body.  The large dog harnesses are made of 1" wide nylon and adjusts from 26"-44" around the body.  For correct sizing, measure the dog around the body, directly behind the front legs. Harnesses are only recommended for walking you dog.  If left on a dog that is unattended, the harness can be easily chewed off.