My body temperature runs cold and I tend to wear thermal underwear much of the winter. There seem to be two categories of thermal underwear  an all polyester, which resembles cotton and a polyester-spandex blend which these are. The all polyester ones cling to your pants make motion a bit harder plus they pill, which makes them look worn out. The polyester  which these are  slide more smoothly under jeans or whatever slacks you're wearing, so they are much more comfortable. Plus they retain a smooth, new look. Heat retention-wise, both compositions seem the same to me. These also dry much faster, if that matters. So, my full endorsement to this option.