Abstract Theoretical underdetermination is a central issue in the Philosophy of Science, having been discussed and debated since the early 20th century. The so-called “Duhem-Quine problem” has been used as an umbrella term to refer to a number of problematic features that arise from the lack of a biunivocal correspondence between theory and evidence. However, the now familiar idea that the detection of an empirical phenomenon is inferred from a complex collection of data (Bogen & Woodward 1988, Woodward 1989, 2000, 2010, McAllister 1997, 2011, Glymour 2000, Harris 2003, Massimi 2007, Leonelli 2015, 2019, Bokulich 2020) entails the recognition that not only theories, but also the description of empirical phenomena are underdetermined by evidence. Empirical underdetermination, understood as the underdetermination of evidence (or assumed empirical phenomena) by data, emerges as a major challenge that has yet to be fully recognized and carefully addressed in the philosophy of science. The paper summarizes the distinction between empirical and theoretical underdetermination as it implicitly appears in the literature to date. It presents them as instances of a more general type, both of which arise from the same basic problems, albeit at different levels and with different implications. Important but often overlooked aspects of the empirical/theoretical distinction, the notion of background assumption, and the different roles of evidence will be clarified.