Despite the popularity of Imre Lakatos’s ideas and numerous references to his Methodology of Scientific Research Programs (MSRP), IR scholars often misstate and misapply his criteria for appraising theoretical development. This article provides a more complete description of Lakatos’s metric, addresses a number of critiques related to its use, and surveys how MSRP has been used to evaluate IR research. It suggests that IR proponents of Lakatos’s methodology could better appreciate the limits of its application, and that those who use his metric could do so in a more informed way. The article argues for a sustained discussion about the promises and difficulties of theory appraisal, and suggests that MSRP may be a useful point of departure for that dialogue. It calls for IR theorists to undertake comparative analyses of different rationalist metrics to provide the basis for making informed judgments about their different strengths and weaknesses in helping to produce better theories.