Religion plays a vital role in the formation of communities and the interaction of cultures, yet is largely ignored in psychological texts. Contemporary religious trends across the globe are rapidly changing. For example, less people are adhering to traditional forms of religious practice, Atheism and secular beliefs are becoming increasingly common and valid, and acts of terror are commonly perceived as motivated by religion. This chapter discusses the operationalization of religion as a variable in scientific research (i.e., religious affiliation vs. use of religion in daily life) before discussing how this operationalization impacts our existing understanding of the relationship between religion and intra- (e.g., coping, personal decision making) and inter-personal (e.g., attitudes and behaviours towards outgroup members) psychological processes. The chapter closes with a discussion of challenges for the future of this field and recommendations for the measurement of this complex variable.