The purpose of this thesis is to give a coherent explanation of the most debatable issue ofrnhighly efficient photoluminescence (PL) in the visible range found in oxidized porousrnSilicon (p-Si). In order to describe the PL mechanism, we considered three competitivernprocesses, process (A) where in both photoexcitation and photoemission occur in thernNSP, (B), in which photexcitation occurs in the NSP and photemission occurs inrnluminescence centers (LCs), in the Si-oxide layer surrounding the NSP, and (C) in whichrnboth photoexcitation and photoemission occur in the LCs. We used the first twornprocesses to explain the optical process by the quantum confinement (QC) and quantumrnconfinement luminescence center (QCLC) models. For the nanoscale Si/SiO2 systems,rnthe radiative recombination rates of processes A and B are compared qualitatively.rnProcess that plays the major role in the PL is determined by the capture cross-section, thernluminescence efficiency, and the density of the LCs and size of the NSPs, all of which arerndependent on the oxidation degree of the p-Si sample. For a nanoscale Si/Si-oxide systemrnwith the LC having certain capture cross-section and luminescence efficiency, it is foundrnthat the higher the LC density and the larger the size of the NSPs, the more beneficial forrnthe QCLCM process to surpass the QCM process and vise versa. For certain LCrnparameters, there is a critical most probable size of the NSPs. In case when the mostrnprobable size of the NSPs is larger than the critical one, the QCLCM process dominatesrnthe PL on the other hand if the most probable size of the NSPs is smaller than the criticalrnone, the QCM process dominates the PL. Furthermore, if the most probable size is closernto the critical one both processes should be taken in to account. In general, for a p-Sirnsample free from oxidation, the QCM process dominates and the model is important torndescribe the PL. For most oxidized p-Si, the QCLCM process dominates and the model isrnuseful to describe the PL and when the NSPs in oxidized p-Si samples have very smallrndensity or very large size, the process that both photexcitation and photoemission occurrnin the LCs in the silicon-oxide layer dominating. Hence, the importance of more than onerntype of mechanism models to describe the PL from oxidized p-Si and NSP embedded inrnSi-oxide is verified