Nuclear magnetic resonance (NMR) and electron spin resonance (ESR) are useful techniques for obtaining information about local structural order and defects in hydrogenated amorphous silicon (a-Si:H). In particular, NMR is a useful probe of the bonding of hydrogen to silicon and of the trapping of molecular hydrogen (H-2) in a-Si:H. ESR is a useful probe of localized electronic states, such as silicon dangling bond defects and optically excited electrons and holes trapped in the conduction and valence band tails, respectively. Recent NMR measurements have shown that H-2 exists in two environments, one clustered and one dilute. The fraction of H-2, which is greater than previously thought, is approximately 10% of the total hydrogen concentration. ESR measurements of the growth and decay of silicon dangling bonds after optical excitation at low temperatures (one probe of the Staebler-Wronski effect) show that these defects are created at a slower rate and anneal at lower temperatures than those created at 300 K. Recent NMR measurements suggest the presence of a paired-hydrogen site, which may be the stabilizing center for the defects that control the Staebler-Wronski effect. ESR results show that the decay of optically excited, band-tail electrons and holes is a universal feature of amorphous semiconductors for which the electron-lattice interaction can be neglected. Although the research is less developed in microcrystalline silicon, NMR and ESR are proving to be useful probes of local order and defects in this material as well. (C) 2002 Published by Elsevier Science B.V.