We review the structural and hyperfine properties of Si dangling bond defects occurring in amorphous SiO2 and at various Si-SiO2 interfaces. These defects have in common a singly occupied orbital on a, trivalent Si Centre. We first briefly summarize the most important methodologies for calculating hyperfine parameters, pointing out their advantages and drawbacks. The properties of the defect centres composed Of Si equivalent to SinO3-n kernels, with n varying from 0 to 3, are then discussed in a,systematic manner. We present three important factors affecting the hybrid state of the Impaired dangling bond: the local geometry around the defect Centre, the electronegativity of the first-neighbour atoms, and the polarization effect due to the oxide environment around the dangling bond. We demonstrate that the cage polarization effect significantly increases the Fermi contact term of the defect Si atom, and discuss the relevant implications. We also quantify the interaction between the dangling orbital and the oxygen atoms belonging to the oxide cage by focusing on the 170 hyperfine couplings. For various defects in amorphous SiO2 and at Si-SiO2 interfaces, we discuss assignments to Si equivalent to SinO3-n structural units in relation to available experimental data. In particular, we address the charge state of the E-y' centre, the peculiar hyperfine properties of the S Centre, and the atomic structures of the three P-b-type centres.