At its most fundamental level, the rule of law requires that everyone, rulers and citizens alike, must obey the law. This type of regime stands in stark contrast to the many regimes throughout history which placed almost no limits upon the powers of those who ruled. We will give a brief account of the development of the rule of law in the West and note the widespread support for the rule of law around the world. We will then present a definition of the rule of law as formal legality, including a statement of the many factors that must be part of a legal system if it is to be accepted as the rule of law. Formal theories of law maintain that if all the formal criteria for law and a legal system are met, then the criteria for the rule of law are met in their entirety. Nothing else (such as whether laws are just) is required. At this point, we will remark upon the problems that this definition presents, especially its moral emptiness and what, if anything, to do about it. We will then discuss the “thicker” and “thinner” substantive theories of the rule of law. The most common criterion for the rule of law added by substantive theory is individual rights. Substantive versions are “thinner” or “thicker” depending on whether they have fewer or more requirements. At the culmination of our discussion of the rule of law, we will briefly note certain important contemporary rule of law problems, the importance of the rule of law in liberal constitutional democracy, and the roles of citizens in supporting and enhancing the rule of law.