Time-dependent density functional theory (TDDFT) is presently enjoying enormous popularity in quantum chemistry, as a useful tool for extracting electronic excited state energies. This article discusses how TDDFT is much broader in scope, and yields predictions for many more properties. We discuss some of the challenges involved in making accurate predictions for these properties.Kohn-Sham density functional theory [1,2,3] is the method of choice to calculate ground-state properties of large molecules, because it replaces the interacting manyelectron problem with an effective single-particle problem that can be solved much faster. Time-dependent density functional theory (TDDFT) applies the same philosophy to time-dependent problems. We replace the complicated many-body time-dependent Schrödinger equation by a set of time-dependent single-particle equations whose orbitals yield the same time-dependent density n(r, t). We can do this because the Runge-Gross theorem [4] proves that, for a given initial wavefunction, particle statistics and interaction, a given time-dependent density n(r, t) can arise from at most one time-dependent external potential v ext (r, t). We define time-dependent Kohn-Sham (TDKS) equations that describe N non-interacting electrons that evolve in v S (r, t), but produce the same n(r, t) as that of the interacting system of interest. Development and applications of TDDFT have enjoyed exponential growth in the last few years [5,6,7,8], and we hope this merry trend will continue.The scheme yields predictions for a huge variety of phenomena, that can largely be classified into three groups: (i) the non-perturbative regime, with systems in laser fields so intense that perturbation theory fails, (ii) the linear (and higher-order) regime, which yields the usual optical response and electronic transitions, and (iii) back to the ground-state, where the fluctuation-dissipation theorem produces ground-state approximations from TDDFT treatments of excitations.In the first, non-perturbative regime, we have systems in intense laser fields with electric field strengths that are comparable to or even exceed the attractive Coulomb field of the nuclei [5]. The time-dependent field cannot be treated perturbatively, and even solving the time-dependent Schrödinger equation for the evolution of two interacting electrons is barely feasible with present-day computer technology [9]. For more electrons in a time-dependent field, wavefunction methods are prohibitive, and in the regime of (not too high) laser intensities, where the electron-electron interaction is still of importance TDDFT is essentially the only practical scheme available. With the recent advent of atto-second laser pulses, the electronic time-scale has become accessible. Theoretical tools to analyze the dynamics of excitation processes on the attosecond time scale will become more and more important. An example of such a tool is the time-dependent electron localization function (TDELF) [10,11]. This quantity allows the time-resolved observation of ...