For use walking / light hiking, they do fog up
My wife uses these goggles for walking and light hiking in the northeast during the winter. They are not used for skiing. In this application they do fog up during certain weather conditions. If there is a strong wind they won't fog. Otherwise, they do. I honestly can't tell the exact weather conditions in which they fog or don't fog. It's sufficiently annoying that we're trying other goggles (Smith I/OS and Smith I/O) which a BC gear expert says do better at not fogging and are even more reliable and better performing than the Smith Phenom goggles with a fan.