A formal, systematic, and rigorous process of inquiry used by nurses to generate and test the concepts and propositions that constitute middle range nursing theories, which are derived from or linked with a conceptual model of nursing. The theories include: Grand: Health belief model; Transactional model of stress and coping; Life process interactive person-environment model; Roy’s adaptation model; Interacting systems conceptual framework. Middle-Range: Theory of self-care deficit; Theory of health promotion; Theory of self-regulation; Theory of uncertainty in illness; Theory of acute pain management; Theory of families, children, and chronic illness. Practice: Theory of interpersonal relations; Theory of representativeness heuristic; Theory of communicative action; Theory of clinical reasoning in nursing practice; Theory of end-of-life decision making.