The humble undershirt has served gentlemen for generations-but there are as many reasons not to wear one as there are reasons in the undershirt's favor.
Undershirts protect your dress shirts from sweat stains by absorbing moisture. They provide warmth in cool weather and can cool the skin in warm weather, and may prevent chest hairs from poking through the shirt.
All too often, the undershirt is visible through a shirt, especially if the shirt fabric is on the thinner end. They can reduce mobility if the fit is poor.
If you wear undershirts, consider a heathered gray to lower its visibility. Always choose undershirts with sleeves, or they won't block sweat and deodorant from your shirt. Undershirts should be close fitting for mobility, and you may prefer a deep cut v-neck so it doesn't show when your top button is undone.