The 4-point adjustable chest harness, independently adjustable back harness, non-slip buckle system, mesh panels, and perforated EVA foam offer a custom, comfortable fit and added breathability. Now with added water resistance, your dogs gear is protected from unpredictable weather, shallow river crossings, and inevitable mud puddles. The increased space of the pannier compartments and the added zipper accessory pockets also mean more room for the K9 essentials. The K9 Pack Large and Medium also fit the 2019 K9 Bed for more comfort in the backcountry. This hiking season, let your dog explore the wild (while carrying his own sh*tsometimes literally). Review the Mountainsmith K9 Pack Guide and sizing below for best fit: Large: best for dogs over 80 pounds with large girth chests. Medium: best for dogs 4090 pounds with medium girth chests. Small: best for dogs 2050 pounds with small girth chests. All Mountainsmith products come with the Forged for Life Guarantee, the official Mountainsmith lifetime warranty.