This thesis is divided into two parts. The first part aims at the determination of basic excitonic properties by considering central-cell corrections. For large principal quantum numbers, the nonparabolicity of the highest valence band is treated. For the analysis of the 1S paraexciton the additional coupling to LO phonons is addressed. The second part examines the absorption edge of the phonon-assisted transition into the 1S yellow orthoexciton and determines the corresponding deformation potential. The deformation potential is used to calculate the phonon-assisted Auger coefficient.<eng>
