This chapter presents an introduction to nursing theory with a discussion of its history, significance, and analysis. A nurse increases professional power when using theoretical research as systematic evidence for critical thinking and decision making. When nurses use theory and theory based evidence to structure their practice, it improves the quality of care. They sort patient data quickly, decide on appro- priate nursing action, deliver care, and evaluate outcomes. They also are able to discuss the nature of their practice with other health professionals. Considering nursing practice in a theory context helps students to develop analytical skills and critical thinking ability and to clarify their values and assumptions. Theory guides practice, education, and research (Alligood 2014, in press; Chinn & Kramer, 2011; Fawcett, 2005; Meleis, 2007).