It is not difficult to predict that the TCAD system VISTA will grow in the future. Consistent extensibility without redesign has been a design paradigm to account for this expectation. The orthogonality of the widget-oriented architecture of the user interface provides a clear concept for future extensions. The generality of the PIF syntax on the data level and LISP on the task level, together with their mutual conformity also enables a variety of interfaces for extension and customization. The application framework architecture itself is ideally suited for adding (specific) functionality in the form of applications, without changing the rest of the system.