In this thesis a newly developed time-dependent renormalized natural orbital theory (TDRNOT) is introduced. After the derivation of the equations of motion, TDRNOT is benchmarked with the help of a numerically exactly solvable model helium atom in laser fields. It is shown that, unlike TDDFT, TDRNOT is able to describe doubly-excited states, Fano profiles in electron and absorption spectra, auto-ionization, Rabi oscillations, high harmonic generation, non-sequential ionization, and single-photon double ionization in excellent agreement with the corresponding TDSE results.<eng>
