I ordered a Medium which fit me very closely (I'm 5'5", 150 lbs). It takes a bit of doing to get on, but once it's on - it works really well. From what I've observed it serves two purposes at the same time. I was a bit sweaty when I put it on at about midday in 33degC Caribbean temperatures. But once I had it on, it helped to wick away perspiration and with a light breeze - it helped to cool me down really well. When I jumped in the sea - which was just cool (not cold) - I was extremely comfortable in the water, with a (Quiksilver) board shorts and (Starter) mid-thigh brief. Of course it keeps the sun burn of your torso too - and you need a whole lot less of sunscreen! Highly recommended.