Tremendous advances have been achieved in microelectronics technology during the past decades. With continuing efforts to improve the speed and functionality of integrated circuits, higher integration densities are forcing device dimensions to decrease to the scale of the wave length of electrons. On the other hand, devices based on pure quantum mechanical effects have been designed, such as resonant tunneling diodes, carbon nanotube based devices, molecular switches, and quantum dots. This implies that in nano-electronic devices quantum effects play an important role. With the aid of numerical analysis one can get a deep insight into the device operation and investigate methods to improve the device performance. The non-equilibrium Green's function (NEGF) has been successfully used to investigate the characteristics of nano-scale silicon transistors, carbon nanotube-based transistors, and molecular devices. Using the NEGF formalism, quantum phenomena like tunneling- and scattering processes can be rigorously modeled. With the aid of this formalism we investigated the behavior of carbon nanotube transistors and RTDs. Based on the simulation results we propose methods for improving the performance of such devices.