Let me say this, its a shame that this is a base layer that will mostly be hidden from view under another layer, this is such a sharp layer! The design is fantastic I just wish the full color skull was also on the front. Cycology always has such amazing designs. I think this is a really good base layer, its not insulated, the fabric is thicker than a jersey but it is not designed as a thermal layer. Its also very clingy, form-fitting without being restrictive. In terms of sizing, I used the online tool but I also went with experience. I am 6'2" and 175lbs and slim. I typically wear slim fit shirts. My Cycology short sleeve jerseys are XL as are my Cycology cotton t-shirts so I ordered an XL (even though the fit guide suggested L) and the fit is perfect! Its exactly what I wanted in terms of fit and comfort, any smaller and it would be too small and snug. This is definitely a new cool weather favorite!