Comfortable harness made of breathe easy mesh material. A lot safer than a collar because the force doesn't go directly on the neck. The pressure is distributed throughout the body. Largest size will most likely fit a dog up to 23 lbs. In order to fully utilize the functionality of the harness, as well as the fit, we recommend measuring the largest part of the dog's chest and following the size chart. If the measurement is between sizes, please size up. For reference, estimated sizing by weight is Small: 5-10 lbs Medium: 11-16 lbs Large: 17-23 lbs.