Stress is an inevitable part of human life, and how we experience and respond to stressors can dramatically influence our health and well-being. Stressors occur in many forms and with varying degree of severity, ranging from minor daily hassles to major traumatic life events. In addition to the demands placed upon us by a stressor, our subjective evaluations can shape emotional, physiological, and behavioural reactions, which in turn can influence mental and physical health. This entry outlines key theoretical models linking stress to health and well-being, gives an overview of major stress-responsive physiological systems, and describes relevant research of the presence and effects of stress across the lifespan.