CamelBak packs work best when the water's on the inside and the outside and cargo are dry. This rain cover fits securely to your CamelBak pack with a Velcro closure and drawstring and will protect it from everything short of a jump in the pond. Use the S/M for the H.A.W.G., Cloud Walker, Trail Blazer and Zephyr. Use the M/L for the Rim Runner, Peak Bagger, Ares, Ethos and Kronos.