It is generally estimated ([79], [80]) that for interactive applications, up to 50 % of the program code is devoted to the implementation of the actual user interface. Experience shows that the overall development effort is often comprised to a comparable extent by the implementation of the user interface. Thus, in the case of TCAD, it is of vital importance to separate the user interface implementation from the respective tool implementation and to foster re-use. It has been shown how this can be done for batch-mode applications. Fortunately, TCAD is not the only area which is influenced by the advent of platforms for Graphical User Interfaces. The hope that a stable, high-level user interface standard will evolve in the near future is not very realistic. Eventually, applications and portable systems will have to continue duplicating user interface functionality in order to maintain stability and portability. This also holds true for VISTA.