A waterproof rain jacket is an essential kit component for hikers, backpackers, and a multitude of other outdoor adventurers. As with any gear category, there are varying levels of effectiveness within it, including breathability, comfort, and durability. The most important measure of any rain jacket, though, is how well it seals out moisture—and the term “waterproof” is more nuanced than you might think. To help you understand waterproofing levels, get up to speed on how membranes work, and how those factors (and other design elements) act in concert to keep you dry on the trail, we’ve put together this handy guide.