nextnano is a simulation tool that aims at providing global insight into the basic physical properties of realistic three-dimensional mesoscopic semiconductor structures. It focuses on quantum mechanical properties such as the global electronic structure, optical properties, and the effects of electric and magnetic fields for virtually any geometry and combination of semiconducting materials. For the calculation of carrier dynamics, two models are currently implemented that provide results for the limiting cases of highly diffusive or purely ballistic quantum-mechanical transport. In this paper, we present an overview of nextnano's present and future capabilities and discuss some key concepts in the areas of code structure, numerical techniques, and electronic structure principles.