What did medieval men wear under their clothes? Medieval women?
In Imperial Rome, both men and women were known to wear simply wrapped loin-cloths, probably made from linen, under their outer garments. In addition, women might wear a breast band called a strophium or mamillare, made from linen or leather. There was, of course, no universal rule in undergarments; people wore what was comfortable, available, or necessary for modesty -- or nothing at all. Individuals competing in sports, like the women depicted in the mosaic shown here, would have benefited from confining garments.
It's entirely possible that the use of these undergarments continued into medieval times (especially the strophium, or something similar), but there is little direct evidence to support this theory. People didn't write much about their underwear, and natural (as opposed to synthetic) cloth doesn't usually survive for more than a few hundred years. Therefore, most of what historians know about medieval undergarments has been pieced together from period artwork and the occasional archaeological find.
Update: A discovery made in Austria indicates that women probably wore garments much like modern-day brassieres and underpants at least as far back as the 15th century. See your Guide's blog, Exciting find in medieval underwear, for more information.
What is known is that medieval men wore underpants that were quite different from loincloths.