The aim of device modeling is to develop a link between materials properties and the electrical device characteristics of a nano-structured solar cell. This is in contrast to materials modeling, where materials parameters (like the optical absorption alpha(lambda), the electrical mobilities mu_e, mu_h of electrons and holes, various relevant energy parameters,��) are studied and theoretically modeled based on physical and chemical phenomena and interactions. In device modeling, we suppose that all materials parameters needed are already known, either from quantitative materials modeling or from experiments. We also will assume that an optical analysis of the cell already has resulted in the knowledge of the optical generation of electron-hole pairs and of excitons, both as a function of position and wavelength. Thus, the scope of this chapter is electrical device modeling of nano-structured solar cells.