Process and device simulation is commonly used for the design of new VLSI technologies. Simulation programs serve as exploratory tools in order to gain better understanding of process and device physics. On the other hand, simulations are also carried out after the design phase to optimise certain parameters of a technology, e.g., to improve device performance and reliability or to increase the yield. For all these tasks the term TCAD, short for technology computer-aided design, was coined. TCAD includes both physically rigorous as well as simplified process and device simulation in one to three spatial dimensions. Furthermore, links to layout-oriented CAD and circuit simulation are required.