Hydrogenated amorphous silicon (a-Si:H) is an attractive material for low-cost solar cells, but the light-induced formation of metastable defects leads to a degradation with time of the conversion efficiency, the Staebler-Wronski effect. The common notion is that the 'dangling-bond' (db) defect, i.e. a singly under-coordinated silicon atom, plays an important role in this degradation. This defect can be detected by electron spin resonance (EPR). The resulting absorption spectrum is characterized by the spin coupling with the external magnetic field (g-tensor) and the hyperfine coupling to the central nucleus of the defect. This thesis analyzes the role of electronic and structural effects on the EPR-parameters of the silicon dangling bond by means of state-of-the-art ab initio computational methods. First, molecular and crystalline (c-Si) db-systems are used for comparison with experiment and for a systematic study of the influence of the local defect geometry on the EPR-parameters. The results of this study are then applied to and contrasted with the computation of the EPR-parameters for an ensemble of structural models of the a-Si:H db. The theoretical EPR-parameters agree well with experiment for selected c-Si db-models. The influence of the local defect geometry on the hyperfine parameters can be related to the interplay between sp-hybridization and spin delocalization. The results for a-Si:H explain why only the hyperfine tensor maintains the uniaxial symmetry, while the g-tensor becomes rhombic (i.e. asymmetric), in agreement with a recent experiment. A closer inspection of the models, as well as a systematic study of strain effects, reveals that the a-Si:H db is a network defect, for which delocalization is important. It is therefore conceptually different from its crystalline counterpart.