There is one main compartment with a very long access zip, and a large top pocket. It also has two extendable side pockets which easily hold large water bottles or a down jacket, although you'd have to be Mr Tickle to access them whilst the bag is on your back.
There's an internal sleeve for a water bladder and all the external loops to hold the drinking tube in place. There's a couple of large grab handles, plus a series of small loops which can be used to clip things to. There's also pole straps each side along with base loops, to stop poles swinging around.
The shoulder straps are thin and vented, but surprisingly comfortable, especially when used with the pocketed belt and chest strap; there's even a mesh pocket on one of them which will take a smart phone. The foam back plate is also vented, although this is more to keep the weight down, than to help your back from getting hot.
The Elastin coated 210D Robic 70D nylon external construction is highly water resistant: The Stir 28 will withstand a strong spring shower, but not 5 hours in a lake district downpour!
This is a great basic piece of kit with clever features, such as elasticated loops to tidy away dangling straps on the waist belt; 'less is unquestionably more' in the case of the Thule 28L backpack!
