If you determine what kind of running you will be doing, as well as what you will be carrying, before you go searching for a new running pack, the process of choosing the one that is right for you will be much easier. Some runners need only a water bottle and perhaps a bottle of energy gel, in which case a waist running pack will be sufficient; other runners may need a larger pack that will be able to stow a light jacket, first aid kit, and other items that may be useful in the woods or on the road.
Hydration packs are a specific design of running pack that features a bladder inside a specially designed compartment. This bladder is attached to a hose that mounts on one of the shoulder straps of the running pack. Runners who will be running primarily in the woods or running for long distances may require a hands-free drinking system with a high capacity; if this is the case, a hydration pack is a great choice. Such packs come in a variety of sizes, and the features of the pack can vary significantly by model. Some of the best ones feature a mesh back and strap system that allows the runner's back to breathe and wick sweat during exercise.
The runner looking for a lighter, less bulky alternative will want to consider a waist pack, sometimes known as a fanny pack. This design of running pack tightens around the waist, leaving the back and shoulders free of any obstruction. In most cases, the waist pack will feature slots for one or two water bottles; the slots should be tilted outward to make grabbing the bottles easier while running. These slots are usually mounted on the rear of the waist belt to ensure the water bottles are not in the way while you are running.
Some waist packs feature additional slots or sleeves in the front of the belt and on the sides. These are useful for storing energy gel packets or tubes, or even a money clip or watch. Choose the size of waist running pack that is suitable for your needs. If you need more storage, choose a belt with more slots; if light weight is the most important consideration, look for a simple but comfortable model that will allow you to carry one or two water bottles and nothing more.