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 evi- dence 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 out- comes. They also are able to discuss the nature of their practice with other health professionals. Con- sidering nursing practice in a theory context helps students to develop analytical skills and critical thinking ability and to clarify their values and as- sumptions. Theory guides practice, education, and research (Alligood 2014, in press; Chinn & Kramer, 2011; Fawcett, 2005; Meleis, 2007).