Building user interfaces with tool boxes has its limitations. Although they allow users to build basic interfaces efficiently on a high level of abstraction, they lack simple extendibility. That is, whenever users have needs for new building blocks, which cannot be composed of existing ones, users have to resort to conventional programming techniques. Agentsheets address this problem by introducing an intermediate level of abstraction between high-level building-blocks and the level of

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... nal programming language. Furthermore, Agentsheets do not only provide the incremental construction of behavior, as it is the case in most object-oriented systems, they also allow the incremental construction of the "look" of artifacts. This paper gives a short introduction to Agentsheets and then elaborates the concepts in form of a case study describing the process of building a new front-end for a commercial expert system with Agentsheets, and testing it. Abstract Building user interfaces with tool boxes has limitations. Most of them allow users to build basic interfaces efficiently on a high level of abstraction. However, when users have needs for new building blocks, which cannot be composed of existing ones, users have to fall back on the low level of abstraction provided by conventional programming languages. Agentsheets address this problem by introducing an intermediate level of abstraction between high-level building-blocks and the level of conventional programming languages. They provide means for the incremental definition of the behavior as well as of the "look" of artifacts. This paper gives a short introduction to Agentsheets and then elaborates the concepts in form of a case study describing the process of building a new frontend for a commercial expert system with Agentsheets.