The first thing you need to consider when in the market for a pair of bike panniers is whether they are compatible with your bike's rear rack. Or, alternatively, you could decide which rear rack to buy based on what brand of bike panniers you like. You also need to consider how much gear you will typically be taking with you, so that you have enough capacity. And, if you plan on going anywhere where there's rain, you have to ensure that your panniers are 100% waterproof rather than merely water-resistant.