I need baselayers for my 13 year old who normally wears Medium (5'8", 140 lbs). We bought the Small from Terramar and it fits perfectly. I recommend going 1 size down or 2 to buy different sizes as returns are free to see what fits you best!I bought the top and bottom from 2 different companies: ABOVE is "Terramar" Terramar Men's Thermolator II Crew, Black , Small is Coldpruf Coldpruf Men Platinum Long Sleeve Crew (Medium, Heather Grey) BOTTOM is "Terramar" Terramar Men's Thermolator II Pant, Black, Small - Coldpruf Men's Coldpruf Platinum Dual Layer Bottom, Black, Medium My son preferred both Terramar pieces. Why that? - Coldpruf was not only large (so take 1 size down as it's the bottom layer) but also very chunky. Doesn't fit the body but more like top layer shirts - No stretch enough - Kä ltefest itches and terramar is soft. sensitive to what he's wearing, and if the material "itches" him, he'll never wear it. He started screaming after 1 minute - "Take it off, it's too itchy". I felt it with my hands and the fabric feels very artificial and scratchy compared to Terramar which is very soft to the touch. Actually I can't get him to take it off and now he wears it at home. Lol Bring back Coldpruf and stay with Terramar. Will be updated after the ride!