It's thermal underwear for different occasions put out by Champion. They have quite a few different lines including one for specifically hunting. Only problem is the hunting one doesn't have a mock zip. www.duofold.com I'm pretty sure a few people from here use it as I recall hearing about it here several times over the years. They also have a polypro line as well and their prices are reasonable. You can actually get them cheaper on sites like www.campmor.com and others.