Nearly 50% of adults have one or more chronic illnesses. Self-care is considered essential in the management of chronic illness, but the elements of self-care in this context have not been specified in a middle-range theory. This article describes a middle-range theory of self-care that addresses the process of maintaining health with health promoting practices within the context of the management required of a chronic illness. The key concepts include self-care maintenance, self-care monitoring, and self-care management. Assumptions and propositions of the theory are specified. Factors influencing self-care including experience, skill, motivation, culture, confidence, habits, function, cognition, support from others, and access to care are described.