Ditto. The t-shirt is a sweat sponge for me.
I wear a t-shirt under any button-up shirt for all but the hottest days (when I wear polo shirts, or t-shirts.)
Never.
My wife does. Sometimes, even in the San Antonio heat, she’ll wear an outfit like the following:
Denim vest
Dress
Undershirt
Bra
I told her that she has no right to complain about the heat when she has four layers of clothing on her chest. 
Only in the styles that require them or for warmth. And, even then, they’re really outer shirts (like heavy tees) worn as undershirts. I don’t have any plain undershirts. The material is just too flimsy.
EDIT: and either kind makes my sweating worse. Plus absorbing sweat makes me stink more.
Only for work. It’s hot in the factory and we all sweat. Many guys wear only t-shirts but I prefer button down work shirts for the extra pocket capacity. Problem is, we all sweat and sweating against the nickel coated button on jeans makes for belly rashes. Tucking in a t-shirt seems to help prevent that, so I wear one, but almost always a goofy saying T and not a real underwear undershirt.
The only time I don’t wear a white cotton t-shirt under my outer shirt is when my outer shirt is a t-shirt.
I am female and for me the answer is no, but back when I wore scrub tops regularly I generally wore an undershirt under the scrub top, especially if it was a pastel colour
Not anymore though, unless I know its going to be cold.
My dad has worn a white cotton t shirt under all button up shirts (dress shirts, sports shirts, long or short sleeves) his whole life. He is 71 now, but thanks for this thread. I have a little note book where I put down things like preferences my parents have if they ever need long term care. This is so obvious I am sure I wouldn’t even clue in until I realized something looked wrong on him.
I’m usually too warm as it is, I don’t need to add another layer.
Male. I wear undershirts under dress shirts, but otherwise not. With dress shirts, the fabric doesn’t feel right against my skin anymore.
At the office, I wear a dress shirt and jeans. During the warmer months, I usually wear a tee that looks good as part of a tee-and-jeans combo so I can just take off the dress shirt when I get to the car.
I can’t stand layers unless it’s cold and I’m trying to keep warm. That doesn’t happen much here. Therefore, no t-shirts. No sweaters, either- why would I want to sweat?
I saw that Raj on the Big Bang theory regularly wears four layers- a jacket over a sweater over a shirt over a t-shirt. I don’t see how anyone could stand that.
Female, and 100% of the time. I wear basic camisoles, and they’ve become such a part of my wardrobe that deciding to NOT wear one would feel akin to deciding not to wear a bra for the day. Which I occasionally wish I could get away with 
Rereading my old thread it’s interesting how many ladies wear a undershirt/camisole. Learned something in this thread. 
Mom finally got dad to stop wearing undershirts after he turned 60. They were required in the Air Force and that’s where he picked up the habit. I’ve tried not wearing an undershirt with Polos and just didn’t like how it felt on my skin. I got to have my undershirt for comfort.
Except for a few weeks in the hottest part of the year, I wear a long sleeved cotton shirt like this one under a mock turtle neck like this one. If it’s over 82 degrees, I might go without the under shirt. In winter I replace the mock turtle with a heavier sweater. My thermostat runs cool.
Attention sweaty people : I recently discovered sweat block, and it’s freaking amazing.
I don’t wear a t-shirt when it is hotter than about 60-70F outside.
I haven’t worn an undershirt since I was a kid.
I wore slips under my dresses way back when.
I wear t-shirts all summer and sweat shirts all winter, if it’s really cold I will wear a long sleeved t-shirt under my sweat shirt. If it’s really really cold I might layer 2 because I hate wearing a coat.
In the rare times that I have to dress up, I have tops with material thick enough that I don’t have to wear anything more than a bra.
That’s funny…sometime after answering “No - never” and 2012 I started wearing tank tops under my t-shirts.
I realized that I did not have to suffer with my stupidly long torso and thus stupidly short t-shirts. I could get some extra-long tank tops from Old Navy and wear those under everything. So that’s what I do now.
I still don’t technically wear t-shirts under t-shirts, but I do wear tank tops under t-shirts and I look a whole lot better (except they do minimize my assets a bit but oh well).
Always. And never a V-neck or tank-top style t-shirt. Even if you can barely see them under a dress shirt, and especially if you can see them well under a white dress shirt, they look completely ridiculous.
I almost always wear one. Sometimes with a polo-type shirt I don’t, but most days I’m wearing a button-down shirt and I always, always do with them. Today I’m wearing a polyester blend polo-style shirt with an undershirt.
Yes. My wife said men are supposed to. I trust her clothing decisions over my own.