Instead of my usual medium in cycling clothing, I am a large in Castelli. I?m 5?9?, 140 lbs. with skinny arms and a 36?-37? chest and the large fits good. The medium looked and felt tiny. Be warned that this vest is also designed to be quite short in the front regardless of the size. So even my size large which looks perfect in the length on the back and through the chest is short in the front. That?s just how this vest is supposed to be. This short length in the front is for when you?re in the aero position so don?t worry about how it looks when you?re standing up. Also, you sacrifice pockets for a mesh back but it?s well worth it. I have a cycling jacket that doesn?t have a mesh back and you get way too hot very fast. You gotta have the mesh back for venting. The Leggero vest is made of an ultra-light material that folds up teeny-tiny. It comes with a palm-sized carry pouch that amazingly holds the vest. However, I don?t use it. I just fold the vest up and put it in a jersey pocket but it?s nice that they included the bag. I got this vest for days that start cool where you?re maybe also wearing arm and knee warmers but will probably take all or some of that off at some point. This vest is perfect for that situation and with it I don?t need to wear a base layer. It?s excellent for the weather we have here in SW Florida. The vest provides decent warmth and wind protection but not too much and it?s easy to stow. My only complaint is that I could do without the two way zipper. I never use it and it causes me occasional annoyance. If we could give half star ratings I?d give the Leggero vest 4 � stars but since that?s not an option I?m going with the 5 stars. To summarize, pros: mesh back, very lightweight, designed for cool (not cold) days, easy to stow, highly visible (bright yellow color). Cons: odd sizing, about an inch or two short in front, two way zipper (although some people might like the zipper). Overall I recommend it especially if you can buy it on sale.