Pressured by globalization and demand for public organizations to be accountable, efficient, and transparent, university rankings have become an important tool for assessing the quality of higher education institutions. It is therefore important to assess exactly what these rankings measure. Here, the three major global university rankings—the Academic Ranking of World Universities, the Times Higher Education ranking and the Quacquarelli Symonds World University Rankings—are studied. After a description of the ranking methodologies, it is shown that university rankings are stable over time but that there is variation between the three rankings. Furthermore, using principal component analysis and exploratory factor analysis, we demonstrate that the variables used to construct the rankings primarily measure two underlying factors: a university’s reputation and its research performance. By correlating these factors and plotting regional aggregates of universities on the two factors, differences between the rankings are made visible. Last, we elaborate how the results from these analysis can be viewed in light of often-voiced critiques of the ranking process. This indicates that the variables used by the rankings might not capture the concepts they claim to measure. The study provides evidence of the ambiguous nature of university rankings quantification of university performance.