The structural realist approach to spacetime theories, explored in Chap.  5, represents one of the more influential third-way spatial ontologies to have recently appeared on the scene, but there are various difficulties associated with this strategy that have the potential to undercut its viability. This chapter will examine what most likely constitutes the primary obstacle, namely, the numerous underdetermination problems that plague structuralist conceptions of spacetime theories, a significant and contentious development in the structuralist literature that, as will be argued, also demonstrates the advantages that the spacetime version of epistemic structural realism (ESR) holds over its ontic competitor (OSR). Recent non-realist structuralist accounts, by Friedman and van Fraassen, have touted the fact that different structures can accommodate the same evidence as a virtue vis-à-vis their realist counterparts; but, while problematic for OSR, and possibly ESR, these claims gain little traction against a properly constructed liberal version of epistemic structural realism (see Chap.  5 for an introduction to these structuralist concepts). Overall, a broad construal of spacetime theories along epistemic structural realist lines will be defended which draws upon both Friedman's earlier work and the convergence of approximate structure over theory change, but which also challenges various claims of the ontic structural realists.