Many backpacks are marketed as anti-theft, meaning it’s tougher for someone else to get inside your pack. Some backpacks have zippers that allow you to open the main interior compartment while you're wearing it—so you don’t have to take it off and risk having someone else pick it up. Others are designed with a webbing over the zipper openings, making it very hard for someone to locate and unzip the bag while you have it on.