Pretty basic longjohns / long underwear / base layer / thermal underwear / whatever - they're a bit scratchy and baggy and look weird under any kind of fitted pants, but I think that's sort of traditional for this particular genre of garment. Good to have around for occasional cold-weather forays, but if you find yourself needing a lower-body-heat-management-solution more regularly, it wouldn't hurt to look into the slightly higher-tech (and more expensive) options.