At first glance, testers questioned the horizontal fly on the breathable CW-X Litefit Boxer Brief ($35, cw-x.com)--or the need for a fly at all. But in the end, they forgot it was even there. (Check out the complete Runner's World Gear Guide.)
Pro: The fit is tight yet comfortable, thanks to a design that offers light support.
Con: Some testers felt that the wide waistband was a little too tight.
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Look closely and you'll see that the Lululemon Premium Technikini ($18, lululemon.com) is printed with inspirational phrases to add a touch of fun to an otherwise utilitarian piece of running gear. (Find out How to Wash Your Running Gear.)
Pro: The extra-wide and smooth waistband earned high marks from our testers.
Con: Stitching at the leg openings can dig into your backside when you're sitting down.
Winter weather can slice through all but the hardiest pants. (Video: Running Shoe Care) But it won't pierce the windproof panel on the surprisingly soft Brooks Thermal Wind Boxer ($35, brooksrunning.com).
Pro: Brushed-fabric interior and long cut provide insulation almost to the knee.
Con: "Thermal" is right; these undies are too warm for anything but freezing conditions.
Made of merino wool and Lycra, the Icebreaker 200 Lightweight Pace Brief ($40, icebreaker.com) is comfy against the skin, yet stretchy.
Pro: This pair breathes well in hot weather, but the temperature-regulating properties of wool keep you warm on cold days.
Con: Some testers felt the gray-on-charcoal color scheme is a bit dull.
For more workout underwear, check out our list of the Best Sports Bras for Women.
"Breezy" best describes the Craft Cool Brief with Mesh ($25, craftsports.us). Breathable panels on the sides offer excellent ventilation, even indoors on the treadmill or in warm weather.
Pro: High-cut leg openings allow total freedom of movement.
Con: To avoid the tighty-whitie look, opt for the black version.
PLUS: Do hot pants really boost workout results? See if this latest trend in workout gear really works.
Testers loved how the Patagonia Active Hipster ($20, patagonia.com) wicked moisture on hot runs. Bonus: It's made of recycled polyester blended with spandex.
Pro: Despite a low-riding fit, the waistband is extra-wide and comfortable.
Con: No matter how many miles they ran, our testers couldn't find anything about the Hipster that rubbed them the wrong way.
Plus, check out some wacky but innovative running gear you haven't heard of, in our list of Weird Gear With Results.
Banish the burn on your next long run with the Asics ASX Boxer Brief ($25, asicsamerica.com). The three-inch inseam is short enough so it doesn't bunch up, but long enough to stop skin-on-skin friction. (See the type of workout that all distance runners need.)
Pro: Stretchy panels snugly hold everything in place without being restrictive.
Con: The tight fit led our testers to feel that the shorts run a bit small.
The Moving Comfort Workout Bikini ($16, movingcomfort.com) may look skimpy, but our wear-testers found it stood up to the task. A generous mesh panel on the back makes this pair highly breathable.
Pro: Seam-free edges at the leg openings eliminate panty lines, even under compression shorts and tights.
Con: None here. Our testers loved 'em.
Related: The Best Workout Gear of 2012
The Ibex Balance Boxer Short ($50, ibex.com) is like a warm embrace on a cold day. Extremely thin and soft merino wool is blended with Lycra for a snug fit that hugs your body while you run. (And before you head out to brave the cold, try this Total-Body Indoor Warm Up.)
Pro: Tightly stitched yarn adds support while eliminating any irritating seams.
Con: The five-inch inseam offers extra coverage, but the legs tend to ride up.
The Brooks Thermal Boy Short ($35, brooksrunning.com) is an ideal base layer for winter runs. Its brushed interior is soft against the skin and provides insulation to keep you warm on the coldest days. (Your fitness doesn't have to suffer when the temperatures dip. Try these Fast Winter Workouts to stay in shape.)
Pro: Compression panels offer mild, but not tight, support while running.
Con: Testers found these too warm for running on anything but frigid days.