In the quantum mechanical treatment of molecules we use the Born-Oppenheimer (adiabatic) approximation, in which the motion of nuclei and electrons is separated. In this approximation the coupling between different electronic states is neglected and nuclei move on a single electronic potential energy surface. Nevertheless, non-adiabatic processes where the coupling between different electronic states becomes large and important. These processes are facilitated by the close proximity of potential energy surfaces, and especially by the extreme case where the potential energy surfaces become degenerate forming conical intersections. Modeling non-adiabatic processes requires accurate calculation of electronic structure states and their coupling. Methods for calculating excited states, the non-adiabatic couplings and conical intersections will be discussed.