Rule-enhanced task models for increased expressiveness and compactness

User centered design and development of interactive systems utilizes theoretically well-grounded, yet practical ways to capture user's goals and intentions. Task models are an established approach to break down a central objective into a set of hierarchical organized tasks. While task models achieve to provide a good overview of the overall system, they often lack detail necessary to (semi-) automatically generate user interfaces. Based on requirements derived from a comprehensive overview of existing task model extensions, improvements and development methods, an approach that integrates logical rules with task models is introduced. By means of practical examples it is shown, that the integration of rules enables advanced execution flows as well as leaner task models thus improving their practical value.