Amongst various formulations of density functional theory (DFT) for electronic excited states, by far the most widely used is linear-response DFT. For historical reasons, that formulation is commonly known as "time-dependent" (TD-)DFT, despite the absence of time in its static, frequency-domain formulation. The latter formalism has a pleasing familiarity for chemists, as it can be cast as an eigenvalue problem in a space of singly-substituted Slater determinants, analogous to the method of configuration interaction with single substitutions (CIS) but incorporating dynamical electron correlation. In favorable contexts, including the electronic spectroscopy of many medium-sized organic molecules, TD-DFT achieves a mean accuracy of ∼ 0.3 eV for vertical excitation energies, which is often sufficient for solution-phase spectroscopy. At the same time, TD-DFT's formal scaling and computational cost are comparable to groundstate DFT, meaning that it is often the only ab initio method for excited states that can address large chemical systems. These considerations have made TD-DFT into the de facto workhorse of computational electronic spectroscopy.