This paper offers an interpretation of Poincaré's conventionalism, distinguishing it from the Duhem–Quine thesis, on the one hand, and, on the other, from the logical positivist understanding of conventionalism as a general account of necessary truth. It also confronts Poincaré's conventionalism with some counter-arguments that have been influential: Einstein's (general) relativistic argument, and the linguistic rejoinders of Quine and Davidson. In the first section, the distinct roles played by the inter-translatability of different geometries, the inaccessibility of space to direct observation, and general holistic considerations are identified. Together, they form a constructive argument for conventionalism that underscores the impact of fact on convention. The second section traces Poincaré's influence on the general theory of relativity and Einstein's ensuing ambivalence toward Poincaré. Lastly, it is argued that neither Quine nor Davidson has met the conventionalist challenge.