Whether or not you skip undies is up to you—panty lines are totally a personal preference—but there are some compelling reasons to consider covering up.
"Bacteria and yeast love a moist and warm environment," says Doerthe Brueggmann, M.D., of Health Goes Female. "So a very effective and simple protection is to wear any underwear under your workout gear to catch sweat and developing moisture. This is why it's not a good idea to go commando!"
However, David Bank, M.D., founder and director of the Center for Dermatology, Cosmetic & Laser Surgery, adds that fabric choice is crucial when it comes to your workout gear. "If you frequently wear bottoms with moisture-wicking capabilities, then underwear is not necessary." And thanks to the latest innovations in fitness apparel, many pants and shorts have built-in undies made from wicking, antibacterial fabric, like the Get Set Go shorts from Under Armour ($40; underarmour.com).