There is an internal sleeve for the reservoir and a place for the hose to exit.

The sleeve is against the inner frame, it has a small plastic hook that holds a 3L camelback bladder perfectly. Although mine broke the fourth day of use (Carry a small carabiner). It's tough to fill as you have to remove it with all your gear in the bag and put it back. There is an elastic strap on the shoulder strap to place the mouth piece when not in use. It's a great pack other than that shoulder straps are made of a gel like foam that are very comfortable, and the pack is highly adjustable.