The electronic properties of semiconductor nanostructures are essentially determined by quantum effects, which determine the discrete nature of energy states, tunneling rates as well as optical transitions. Therefore a quantum transport theory is indispensable for the description of such structures (although some estimates can already be obtained from a semiclassical heuristics). Due to the small dimensions of semiconductor nanostructures even small bias voltages can drive such structures far from equilibirum. Nonequilibrium Green function techniques allow for a consistent description of these devices in particular for stationary states. I will give an outline of this theory and discuss its strength as well as limitations. Particular applications towards the transport through superlattices and quantum dots are given.