During the latest decade, empirical research on the causes and consequences of the rule of law has expanded and, in the process, become extremely influential. However, we show that a number of widely used indices of the rule of law are not interchangeable. This lack of interchangeability is reflected in the fact that they are based on different defining attributes, to some extent cover distinct empirical scopes, do not correlate highly with each other, and support different explanatory factors. Until a consensus has been established with respect to the conceptualization of the rule of law, scholars are thus not free to opt for the measure that fits their data requirements best regarding spatial and/or temporal scope. Instead, they must carefully assess the content validity vis-à-vis their stipulated definition of the rule of law. Given the amount of money and time poured into the rule of law agenda, the problems identified reflect the lack of maturity of ‘good governance’ research.