I use my 3-season REI Passage 2-person tent for four season camping. I've camped in sub-zero temperatures and endured heavy snowfall. One of the problems using a three-season tent in winter weather is that it tends to have a lot of mesh, and when the rain fly is staked out, winds and snow easily blow into the tent. Also, a staked-out rain fly tends to collect snow and may collapse the tent. To counter these problems, I don't stake the rain fly; rather, I secure it close to the sides of the tent, draping it over the tent rather than having it taunt. If there's wind, I use my panniers and firewood to keep the fly in place; in snowy conditions, I pile snow around the bottom of the fly. I found that my tent is narrow enough so that falling snow will slide off the fly. Likewise, cold breezes don't blow into the tent. To help raise the temp. inside the tent, I've used a three candle UCO lantern. Of course, I put it out before going to sleep. One thing to remember about winter camping: you lose a surprising amount of heat through the ground. So in addition to having a cold weather sleeping bag, you should also have high quality, insulated sleeping pad. It's also not a bad idea to place a foam pad underneath an inflatable sleeping pad--that really helps reduce heat loss.