This thread seems to start every few months. I remember searching some similar question and finding about a dozen of these threads. Every time, each side gets entrenched and calls the other side a bunch of disgusting slobs. I have no idea why some people care so much about why other people wear or don't wear undershirts. Personally, I do wear them, in summer because I live in DC and 20 seconds outside is enough to get you sweating. In winter because they're warmer, I'll still sweat some especially if wearing a coat on my way to work, and having worn them for a while I just feel more comfortable in them. To some extent, to prevent chest hair from showing through thinner or lighter shirts. I wear light gray undershirts to prevent the shirts themselves from being seen.