The Celsius jacket is a nice piece of clothing. In moderately cold weather, I'll wear one of their next-to-skin layers (wicks great--and that's actually one of the secrets to staying warm), the '07 Mountain Shirt, the vest and then the Celsius. But when it gets really cold, that's when you'll need another layer under the Celsius but on top of the underlayers. I hope to buy some of the Kelvin insulated line, maybe next year. In the meantime, I can either use the 90% under this or in super cold, I'll use some of my old Winona as an insulative layer. Nice thing about Winona is that it too stretches and form fits much like Sitka, as opposed to a lot of heavier stuff out there which is simply too bulky for what I want to do with it.