The purpose of this chapter is to explicate how theories (practice, middle-range and grand) are discovered or generated through research. How theories are constructed is described, and an example of a middle-range theory The Omnipresence of Cancer, is presented. A key intention within the chapter is to explain how theories guide research, education and practice. It is recognized that theories, describe, explain, predict and control research, education and practice (Fawcett, 1993).