I use a 3 litre Osprey bladder in my Stratos and Kestrel packs. See Osprey website. Fits neatly into pack hydration pocket, easy to clean, small strap in top of hydration pocket fits through slot in bladder to prevent it slipping down too far so tube stays at ideal length. Comes with magnetic attachment that fits onto rucksack strap. Once you get it adjusted properly magnet in mouthpiece pretty much drops into place when not being used without fiddling. Bite valve has 90 degree open/shut so it will not leak when stowed. Mouthpiece also comes into 3 pieces for easy cleaning. Very happy with it.