Contextual development differs from traditional user interface development in several ways: It focuses on the context of usage and the user population rather than on the technical features required for interaction. However, the latter come into play when transforming context specifications into user-interface code. Contextual development also considers design to be a non-linear process based on activities (re)engineering work processes rather than performing traditional software-engineering tasks. Consequently, contextual development requires usage-relevant (re)presentation and execution mechanisms. Although the specification of task-and user-knowledge has been recognized to be crucial for contextual user interface design, seamless development support is still lacking. The reported TA-DEUS (Task Analysis/Design/End User Systems) project targets toward an environment that allows contextual and seamless design and prototyping based on user and task knowledge. Due to its model-based nature TADEUS is open with respect to diagrammatic notations for specification, and different interaction platforms. As a result, different perspectives on the context, development process and its results can be kept consistent throughout development.