This chapter introduces the role and importance of theory within a research project. The idea of a theory and the concept of a model are explained and their role within research highlighted. This is followed by an outline of the importance of identifying an appropriate model or theory for a research project and how to develop a subsequent conceptual framework for a study. The chapter then introduces the concepts of reliability and validity, describing the relationship between the two, and outlines their importance within a quantitative research project. Finally, the chapter discusses the terms reliability, credibility, trustworthiness and rigour, and their importance within a qualitative research project.