This vest employs evaporative cooling to keep dogs comfortable during the warm summer months. The vest is composed of three layers of polyester that combine hydrophilic and hydrophobic fibers that, when soaked in water and worn, conduct a dog's body heat outward while retaining cool water which is released slowly through evaporation, providing cooling for up to two hours, depending upon temperature, humidity, and your dog's activity level. The outer air mesh layer dissipates a dog's body heat through evaporation, the middle layer absorbs and stores the cooling water, and the inner layer cools the dog. Generously cut to provide a relaxed, custom fit, the vest slips over a dog's head and attaches across the breast, and adjusts to a dog's exact girth using the included buckles. The vest reflects sunlight, heat, and ultraviolet rays. Please specify size according to your dog's girth (measured at the widest part of the rib cage): XXS (12"-19"), XS (17"-24"), S (23"-30"), M (27"-32"), L (31"-40"), and XL (35"-48").