SIESTA's intention is to offer basic modeling components, so called models, which enable its users to formulate their TCAD problems without limitations by the TCAD environment. Moreover, users should be able to accomplish that by means of their favorite and accustomed TCAD simulation tools. SIESTA offers three sorts of modeling components which can be used in conjunction to describe sophisticated TCAD tasks: models for the representation of linear sequences of simulation tools, for simulation networks, and for data manipulation.