Religious fundamentalism has risen to worldwide prominence since the 1970s. We review research on fundamentalist movements to learn what religious fundamentalisms are, if and why they appear to be resurging, their characteristics, their possible links to violence, and their relation to modernity. Surveying work over the past two decades, we find both substantial progress in sociological research on such movements and major holes in conceptualizing and understanding religious fundamentalism. We consider these weaknesses and suggest where research might next be directed.