Estimating whether the Earth’s biota is in the middle of a crisis relies heavily on comparisons between present and past data about biodiversity or biodiversity surrogates. Although the past is a crucial source of information to assess the severity of the current biodiversity crisis, substantive conceptual and methodological questions remain about how paleodiversity and biodiversity are to be properly compared. I argue that to justify claims of a current biodiversity crisis is harder than it appears. More precisely, I claim that (...) paleodiversity measurements are incommensurable with contemporary measurements, given the different ways that biodiversity is conceptualized and operationalized. I conclude by proposing three possible ways of overcoming this incommensurability problem. (shrink)