The investigation of spiritual/religious factors in health is clearly warranted and clinically relevant. This special section explores the persistent predictive relationship between religious variables and health, and its implications for future research and practice. The section reviews epidemiological evidence linking religiousness to morbidity and mortality, possible biological pathways linking spirituality/religiousness to health, and advances in the assessment of spiritual/religious variables in research and practice. This introduction provides an overview of this field of research and addresses 3 related methodological issues: definitions of terms, approaches to statistical control, and criteria used to judge the level of supporting evidence for specific hypotheses. The study of spirituality and health is a true frontier for psychology and one with high public interest.