Stress as a stimulus is integral to dynamic homeostatic functioning. However, evidence of its potentially deleterious effects on health is mounting. The impetus to understand the mechanisms that underlie stress-related negative health outcomes and prevent the development of stress-related disorders has never been greater. Symptom severity and subjective levels of stress, although frequently assessed in studies of stress in nursing research, may not provide adequate data to fully understand the pervasive effects of chronic or overwhelming stress associated with stress disorders. The measurement of stress hormones such as cortisol can help identify bodily changes that are stressor specific, people at risk for development of stress-related disorders, and the efficacy of interventions aimed at stress reduction. Cortisol, as the peripheral output of one of the major stress response systems, possesses several properties that make its measurement highly useful for investigations of stress. This article discusses some of the biological mechanisms involved in the stress response, why cortisol is commonly measured, and issues and approaches in cortisol measurement.