This chapter discusses theory, research, and policy in detail, beginning with what each one is and how it is relevant to the criminal justice systems (CJSs), and then examining the ways that these three elements are connected. Theories are how people make sense of the world and explain things that occur. It provides abstract, conceptual propositions about why things happen. Theories can be evaluated for how testable they are; the development of empirical research questions that help ascertain whether a particular theory is supported is where research begins. Research articles typically follow a predictable pattern, beginning with an introduction that lays out the nature of the study, the research question(s), and the hypotheses. Although the discipline of Criminal Justice and Criminology (CJC) has a substantial body of theory and research about criminality, crime control and criminal justice systems, it has not driven criminal justice policy as significantly as it should have.