There now a large variety of methods exists for the elaboration of nanoobjects and nanodevices. At the same time the reduction in size together with the improvement of theoretical techniques and computational power should allow efficient quantitative simulation of these objects. Such predictive simulation is highly needed since it can serve as a useful guide to build new nanostructures with the desired properties. The aim of this talk is than to give an overview of what has already been achieved in this rapidly evolving field, what can be done at present and what is expected for the future. The focus will be mostly on semiconductor nanostructures and especially silicon.