When I started hiking, pants and underwear were near the bottom of my shopping list. I was too busy snapping up hydration packs and low-rise hiking shoes. Then I came home after a sweltering 12-miler with a burning in my crotch that nothing to do with getting lucky.
It was the Dread Chafe -- the soft skin of the inner thighs rubbed raw after six hours of contact with seams, folds and fabric built for an afternoon on the couch, not a day on the trail. I'd have traded 12 Camelbaks for anything to keep the burn at bay.
Fortunately there's a very easy fix: just buy boxer briefs -- the body- hugging kind, with a two-inch inseam. I've bought these from a range of brands at a range of prices: Patagonia's Capilene boxer briefs are the best I've worn and Under Armour's come in a close second, but I've also bought much cheaper ones from Jockey and department-store brands that worked almost as well.
With underwear, the trick is to avoid cotton and stick with quick- drying, breathable synthetics or Merino wool. Guys prefer extra elastic around their naughty bits while women need undies that won't ride up; many find so-called "boy shorts" do the trick, but it's not a universal fix. It's a matter of what works with your body shape.
Hiking in "technical" underwear frees you to wear non-technical pants. Any comfy old pair of Levis will be plenty tough enough for hiking, bouldering or rock scrambling. Until the first time you get caught in a downpour and spend the rest of your day damp, shivering and miserable because cotton denim takes so long to dry out.
Hence the vast selection of hiking and climbing pants. Most use quick- drying synthetic fabrics and have a bunch of extra pockets; convertibles turn into shorts after you zip the legs off just above the knee (I find that I almost never do this on a hike; but often I'll just zip the legs off in May and leave 'em off till fall.)
The Rolls Royce versions of these pants from ArcTery'x, Mountain Hardwear and other premium brands use stretchy fabric for flexibility. I wear slightly less-sexy pants from Columbia Sportswear and Royal Robbins that are lightweight, durable, functional and not nearly so spendy. I can say that the Supplex nylon in Royal Robbins' pants is simply wonderful: soft like cotton but much lighter and more breathable (though not quite as quick-drying as Columbia's nylon).
Recently Columbia joined Ex Officio in adding anti-bug treatments to their fabrics, like the Bugsaway Baja Long Sleeve Shirt. This is worth considering if you live in very buggy areas; just bear in mind the treatment eventually washes out.
Other common features in technical pants:
Speaking of grit: While you might think hiking in shorts makes the most sense in summer, wearing long pants keeps pebbles and bits of trail detritus out of your shoes, which makes for much more comfy hikes. Also, pants protect your skin from briers, brambles and many bugs.
Pants and undies don't get the attention lavished on the latest, greatest tents and backpacks, but finding the right ones can be every bit as critical to a happy hike.