In this article, the author discusses the tensions between science and religion. According to recent books--many of them by prominent biologists--the era of deference to religious belief--belief without evidence--is ending as faith is subjected to gimlet-eyed scrutiny. Like Mark Twain's celebrated comment about stopping smoking, scholars have found it easy to explain religion: They have done it hundreds of times, in psychological, psychoanalytic, sociological, historical, anthropological, and economic terms. Biologists, by contrast, have been Johnnies-come-lately. But growing numbers of them are exploring the evolutionary factors--the likely "adaptive significance"--of religion. Indeed, given that it is a cross-cultural universal and has had such powerful effects on human beings (for good and ill), religion is an especially ripe topic for biologists' attention. It is both a fertile field and a frustrating one. On the one hand, religious belief of one sort or another seems ubiquitous, suggesting that it might well have emerged, somehow, from universal human nature, the common evolutionary background shared by all humans. On the other hand, it often appears that religious practice is fitness-reducing rather than enhancing--and, if so, that genetically mediated tendencies toward religion should have been selected against. It seems inevitable that both science and religion will persist, often struggling, rarely cooperating, warily cohabiting within people's minds no less than their works, testimony to what underlies the current efflorescence of science/religion books. (Contains a list of 11 resources.)