Atheists represent an inconspicuous minority, identifiable only by their disbelief in God(s). Despite being highly stigmatized and disliked, until recent scientific endeavors, little has been known about this group including why they don’t believe, how many people are atheists, and why they trigger intense reactions. Thus, this paper aims to synthesize what is known about atheists (so far) and to help explain the widespread negative attitudes and prejudice towards atheists; the possible cognitive, motivational, and cultural origins of disbelief; and the unique challenges facing the study of religious disbelievers. To do so, we will explore current findings in psychological research on atheism by considering the complex interactions of cultural learning, motivations, and core cognitive processes. Although significant scientific progress has been made in understanding the factors underlying atheism, there remains much to be explored in the domain of religious disbelief.