The fabric is all synthetic, so doesn't breathe, which is just fine in my freezing cold office environment (kept me nice and warm) but left me a bit too warm for a while after walking several blocks outdoors in a winter coat. It is a looser fit, so my go-to medium fit fine but on the looser side. I might have gotten away with a small, but ordered online so can't say for sure. Good price on sale! Happy with the purchase.