When the user interface (UI) has to be specified, a picture is worth a thousand words, and the worst thing
one can do is to attempt writing a natural language specification for it. Nevertheless, this practice is still
common, and it is therefore a difficult task to move from text-based requirements and problem-space
concepts to a final UI design, and then back again. Especially for the specification of interactive UIs,
however, actors must frequently switch between high-level descriptions and detailed screens. In our
research we found that advanced UI specifications therefore have to be made up of interconnected
artefacts that have distinct levels of abstraction. With regards to the transparency and traceability of the
rationale of the UI specification, transitions and dependencies must be visual and traversable. We
introduce a model-based UI specification method that interactively integrates interdisciplinary and
informal modelling languages with different fidelities of UI prototyping to an interactive design
rationale. With an innovative experimental tool we assemble models and design to an interactive UI
specification. With a zoomable user interface (ZUI) approach, we can visualize the modelled