Religion and violence share a complex and enduring history. Viewing violence and religion from an evolutionary perspective situates both within a broader framework of aggressive, affiliative, and signaling behaviors across species. In this work the authors review genetic, epigenetic, and environmental factors that influence violence, distinguishing two types of aggression that differ in underlying physiology and intent. The use of communicative signals to delimit aggression across species is surveyed and the emergence of human symbolic ritual as a signaling system for creating alliances and promoting in-group cooperation is proposed. Using Wallace’s typology of religion, this Element explores differences across religious systems in relation to socioecological variation and examines the underlying mechanisms by which religion ‘works’. The use of violence as both an ‘honest signal’ and a mechanism for inculcating religious belief is discussed, and the use of religion to incite, validate, and justify violence is reviewed.