Oxide-assisted growth (OAG) via thermal evaporation is introduced to produce large-quantity, high-purity (no metal contamination) silicon nanowires. OAG is a generic synthetic method that can produce a host of one-dimensional semiconducting nanowires, including those of Group VI (Ge, C, SiC), III-V (GaN, GaAs, GaP) and II-VI (ZnO, ZnS, ZnSe) elements. Silicon nanowires are produced with controlled diameter, desired orientation or pattern, and morphology (wire, chain, ribbon, cable). The structural, optical, electronic, and chemical properties of silicon nanowires have been characterized. Atomically-resolved STM images revealed detailed atomic structure of Si nanowires, while STS measurements demonstrated quantum size effect in the bandgap of Si nanowires. Regular arrays of intramolecular junctions in Si nanowires are shown to exhibit sharp conductivity changes across junctions. Si nanowires give strong polarized green-red emission, and exhibit interesting chemical and sensing properties. We further show properly assembled nanowires possess strong photoluminescence and lasing properties. The results offer exciting opportunities for research and applications in nanoscience and nanotechnology.