This article examines underdetermination as a key theoretical assumption in an emerging body of educational research. Underdetermination is described as a broad philosophical position that assumes that social, scientific and technological phenomena cannot be reduced to linear relationships between antecedents and consequences, for instance through the canonical progression from scientific hypotheses to experimentation and then empirical truths. Rather, phenomena are underdetermined by constellations of social and material influences that make the choice of univocal explanations problematic. The principle of underdetermination is implicit in a recent strand of educational research that critiques orthodox interpretations of scientific practices, innovation processes and policy dynamics, recasting them as social, material and political "assemblages". In the article, I analyse the philosophical and epistemological tenets of underdetermination, in order to clarify its nature as a "first principle" in this emerging body of research. By doing so, the article brings into view a broader theoretical debate that has great bearing on future research efforts. The article critically considers the continued theoretical relevance of underdetermination, whilst acknowledging critical arguments mounted against it, namely ontological relativism and political weakness. Some supplementing theoretical ideas are explored in the conclusion.