The number of studies supporting beneficial effects for religion on mental health, physical health, and mortality is growing, but no biological mechanism for such an effect has been proven. The editors propose that religion may have a beneficial effect on immune function through the neuroendocrine system’s modulation of the stress response. Several chapters address this hypothesis by examining neuroendocrine and immune function factors in cancer, autoimmune disorders, wound healing, HIV infection, and other infections. Although there are some studies supporting the editors’ hypothesis, clearly there is a long way to go.