Base layers are a critical component in my clothing system- if you can call it a system. The two primary features I look for are moisture transportation and insulation- but under Armour's stink-proofing called ArmourBlock™ is also nice for mitigating confined space living violations. The interior of the Base 2.0 fabric has a waffle pattern which seems to trap heat well, but also breathes when working hard. This isn't under Armour's compression fitted stuff, but still fits snug. Black is the only color I use for heat retention. For extreme cold, go for the Base 3.0 , but for skiing, slogging, or active sports, this stuff is freakin' great.

Weight: pretty light