Features Two 17 fl. oz. Quick Stow flasks are lightweight and they collapse down when empty; classic one-piece silicone bite valve offers rapid hydration on race day 3D ventilated mesh offers multidirectional airflow and cushioning to maximize comfort and breathability Conveniently located cellphone pocket allows quick access and includes a zipper closure for security Dual adjustable sternum straps increases stability and offer a range of adjustments Stretch overflow pocket lets you toss in a rain shell or stash extra gear Harness storage offers organization for fuel and gear, and includes Quick Stow flask pockets Reflectivity on front and back for early morning or evening runs Sizes are based on chest circumference: Medium = 32 - 40 in. Used once, like new condition, highly rated