Hydration packs allow you to carry liquid in a bag on your back. A tube from the water bag then gives quick access to have a drink without having to stop walking, climbing or cycling. Generally the bags can carry from 1 litre to 8 litres of liquid and can be stored in a normal backpack or a purpose built hydration bag.
Camelbak Mule Hydration PackFree! Read more
Source Low Profile Widepac™ hydration systemFree! Read more
The North Face Animas 12Free! Read more