The rule of law is an essentially contested concept. This means that any scholar who wishes to do research on the rule of law has to decide which defining attributes to include and which to exclude. Some scholars favour a ‘thick’ or maximalist approach where the multidimensional nature of the concept is appreciated. This chapter instead advocates a ‘thin’ or minimalist view. This approach increases the ability to analyse causes and consequences of the rule of law because it avoids conflating it with neighbouring concepts such as state capacity and democracy. The thin approach can be done either by teasing out an undisputed core meaning of the rule of law or by interchangeably investigating different defining attributes. The chapter comes down in favour of the first solution and argues that the best candidate for an undisputed core is ‘formal legality’, that laws are general, prospective, clear, certain, and consistently applied.