Skepticism is about doubt, and doubt is everywhere in the world around us today. There are doubts about whether man-made climate change is real, whether vaccinations are harmful, whether we can trust our politicians or our media, and so on. When is such skepticism warranted, and when does it stray into unreasonable territory? How widespread can such skepticism get while still being coherent? How might a radical skepticism have pernicious social consequences, such as by leading to relativism (and just what is relativism, and what is problematic about it)?