The associations between physical and psychological health and being an informal caregiver are well established. In this article, "caregiving" denotes care that is provided by a family member or friend rather than by a professional who is reimbursed for services. Clinical observation and early empirical research showed that assuming a caregiving role can be stressful and burdensome. Caregiving has all the features of a chronic stress experience: It creates physical and psychological strain over extended periods of time, is accompanied by high levels of unpredictability and uncontrollability, has the capacity to create secondary stress in multiple life domains such as work and family relationships, and frequently requires high levels of vigilance. Caregiving fits the formula for chronic stress so well that it is used as a model for studying the health effects of chronic stress. This article discusses the aspects of the caregiving experience that account for the physical and mental health effects. (Contains 2 tables.)