This PhD thesis reports on the synthesis and characterization of silicon-based nanoparticles. Adequate samples for single particle measurements using laser-induced pyrolysis in combination with the facilities of a molecular beam machine were prepared in order to clarify the origin of the photoluminescence (PL), namely quantum-confined PL or defect-based PL, in individual silicon nanocrystals (Si NCs) and silica nanoparticles (SiO2 NPs). Using confocal scanning laser microscopy with higher-order laser modes, the orientation in space of the nanoparticles transition dipole moment (TDM) and its dimensionality were visualized. For polymer-embedded particles it is found to be linear indicating that localized defect states in the oxide shell determine the PL properties. The spectral analysis of polymer-embedded single SiO2 NPs and Si NCs revealed double-peak spectra consisting of a narrow zero-phonon line and a broader phonon band. In contrast, for free standing Si NCs a one-dimensional as well as a three-dimensional TDM is observed indicating that both mechanisms, defect-related and excitonic PL, are operative. Additionally, PL curves of free standing Si NCs were observed which are clearly related to quantum-confined PL. Ge-doped Si NCs and Ge-doped SiO2 NPs were synthesized and their structural and optical properties were characterized as a function of the germanium content. The non-agglomerated Si1-xGex NCs are made up of a high purity single-crystalline core surrounded by an oxide layer. In accordance with theoretical calculations, the terminal radiative decay rate is found to be faster with increasing content of germanium. Spectral- and time-resolved PL spectroscopy on amorphous Si0.95Ge0.05Ox NPs revealed that several radiative point defect centers contribute to the multiple peak PL. Coexisting non-bridging oxygen hole centers and oxygen-deficient centers in the oxide network are identified to be responsible for the observed luminescence properties.