An International Review of Empirical Research on the Psychology of Fundamentalism is a sequel to Williamson (2020), who examined the sociohistorical emergence of fundamentalism and controversial conceptions of the movement that have dominated decades of empirical research in psychology. He concluded by calling for a critical review of this sizable literature, amassed from the early 20th century. In the present book, W. Paul Williamson and Sarah Demmrich respond by providing summaries and critical observations for 365 empirical studies, collected and organized from peer-reviewed journals. A summary of findings indicated that the largest share of statistical associations between study variables and fundamentalism was moderate in size, followed by those that were weak, and then by strong relationships, which were much less frequent. However, this observed pattern of relationships, particularly those characterized by moderate and especially strong associations, much reflected the findings from sexual bias studies. Finally, the authors offer critical considerations for sample selection, methodology, and theoretical applications in future fundamentalism research.