Heterogeneous systems consisting with nanomaterials (hereafter referred to as nanointerface systems) are extensively investigated in relation to interests in batteries, photo-and electro-catalysts, solar cells, and optoelectronic devices. To efficiently design these functional materials, it is required to obtain atomic-scale insights into the response mechanism to light and voltage bias. However, first-principles theoretical studies on nanointerface systems under light and voltage bias have been scarcely performed because of two problems. Firstly, a huge computational cost is needed to calculate a nanointerface system with a first-principles computational method. Secondly, it is difficult to theoretically describe electronic structure explicitly considering light and voltage bias. In this review, we report the recent progress in our theoretical and computational studies on nanointerface systems. The optical response of various systems such as a gold-thiolate nanocluster and a MoS2-graphene heterostructure has been simulated using a first-principles computational method for carrying out massively parallel calculations of photoexcited electron dynamics. The computational results have been analyzed with theoretical formulas for revealing the role of the interface region in the optical response. We have also developed an original theoretical method for investigating electrode systems. The developed method has been used to elucidate the mechanism of the electronic structure change inherent in nanointerface systems by applying bias voltage, which causes the electronic charging and generates the electric field from a gate electrode.