  The present tutorial provides an overview of the time-dependent configuration
interaction singles (\acs{TDCIS}) scheme applied to nonlinear ionization over a
broad photon-energy range. The efficient propagation of the wave function and
the calculation of photoelectron spectra within this approach are described and
demonstrated in various applications. Above-threshold ionization of argon and
xenon in the extreme ultraviolet energy range is investigated as an example. A
particular focus is put on the xenon $4d$ giant dipole resonance and the
information that nonlinear ionization can provide about resonance substructure.
Furthermore, above-threshold ionization is studied in the x-ray regime and the
intensity regime, at which multiphoton ionization starts to play a role at hard
x-ray photon energies, is identified.
