Although there are several methods to obtain excited states in density functional theory (DFT), the most widely used is the linear-response (LR) formulation of time-dependent DFT, which we call LR-TDDFT here but which is so ubiquitous in modern quantum chemistry that it is usually just called "TDDFT". Its computational cost and storage requirements scale as O(no 2 v 2 ) and O(nov), respectively, where o and v are the number of occupied and virtual orbitals and n is the number of excited states. That number is prohibitively large for core-level excitations unless an active-space approximation is invoked, including only a few core or- * herbert@chemistry.ohio-state.edu bitals along with the full virtual space, such that core-tovalence excitations appear as the lowest-energy states in the spectrum. (In principle, interior eigensolvers could be used for core-level excitations but that approach has not been widely used.) In many-body theory, this activespace approximation is known as "core/valence separation" (CVS), whereas in LR-TDDFT it has also been called a "restricted excitation window", and it amounts to freezing most of the occupied orbitals. For Kedge transitions (i.e., those originating from 1s orbitals in the occupied space), this approximation introduces errors of ±0.02 eV provided that high-quality basis sets are used, although larger errors have been suggested in complete active space self-consistent field calculations. It is less clear what the errors might be for L-or Medge excitations that originate from orbitals with higher principle quantum number, which may not be energetically separated from other valence orbitals. Furthermore, hundreds of states may be required to compute near-edge x-ray adsorption fine structure (NEXAFS) spectra.