University rankers are the subject of much criticism, and yet they remain influential in the field of higher education. Drawing from a two-year field study of university ranking organizations, interviews with key correspondents in the sector, and an analysis of related documents, I introduce the concept of "weak expertise." This kind of expertise is the result of a constantly negotiated balance between the relevance, reliability, and robustness of rankers' data and their relationships with their key readers and audiences. Building this expertise entails collecting robust data, presenting it in ways that are relevant to audiences, and engaging with critics. I show how one ranking organization, the Times Higher Education (THE), sought to maintain its legitimacy in the face of opposition from important stakeholders and how it sought to introduce a new "Innovation and Impact" ranking. The paper analyzes the strategies, methods, and particular practices that university rankers undertake to legitimate their knowledge--and is the first work to do so using insights gathered alongside the operations of one of the ranking agencies as well as from the rankings' conference circuit. Rather than assuming that all of these trust-building mechanisms have solidified the hold of the THE over its audience, they can be seen as signs of a constant struggle for influence over a skeptical audience.