The dimensions of electronic devices are rapidly decreasing and there is a need for a new generation of modeling tools that can accurately calculate the electrical properties of devices where atomic scale details and quantum effects are important. A promising framework for such calculations is density functional theory within the non-equilibrium Green's function formalism (NEGF-DFT). In this paper we present the basic framework and applications of the formalism. The applications include the calculation of the I-V characteristics of a single molecule connected with gold electrodes and the spin-dependent electron transport through a magneto-tunnel junction consisting of MgO layers sandwiched between Fe electrodes. For the formalism to be applied in semiconductor device modeling it needs to be able to handle many thousands of atoms. We discuss new developments and future aspects of the method important for semiconductor device modeling; in particular we show that for important classes of systems the approach scales linearly with the system size.