Belay jacket Bottom layers are a bit simpler, and rarely will you need an insulating layer on your legs. Just put on your underwear and a shell pant over it. I've omitted outlier situations, such as mountaineering 8,000-meter peaks or traveling in polar regions. You'll be looking at very expensive, specialized equipment that you'll rarely use elsewhere. Base Layers Photograph: Backcountry Long johns, tights, or whatever you want to call 'em, base layers are always going to be on your body, so they need to be thin and snug. Your best choices are merino wool and synthetic fabric. Wool keeps you warm even when it's wet, but it takes longer to dry out. Having once been a wool devotee in my early years of hiking, I've switched to synthetics because they dry out easier.