Simulators, such as MOSES [17], SENECA [32] or our SIMON [P1][P2], are circuit level simulators. For them a tunnel junction has two parameters, a resistance and a capacitance, and in the best case an externally calculated energy spectrum is fed into the program to consider discrete energy levels. Thus, the next major step in the improvement of SET simulators, which will lead to an expanded and deepened understanding, is to consider geometrical details and material properties. In analogy to existing TCAD software for conventional devices, where process simulation produces geometry and material related input data for device simulation, one must develop similar methods for SET simulation. Big parts of existing process simulators could be reused. For novel granular production techniques new process simulators have to be devised. Three-dimensional Schrödinger equation solvers that will give energy level spacing and transmission probabilities are mandatory for a more accurate prediction of device behavior. Particularly important will be the accurate computation of capacitances in three-dimensional structures, since single-electron devices are extremely charge sensitive, which makes them strongly dependent on stray capacitances and trapped charges. Issues such as phonon assisted tunneling, electron-electron interaction, self heating, trapping-detrapping and charge relaxation must be addressed and modeled. With smaller structures which are manufacturable year by year, the need for precise analyses tools will grow.