In architecture various approaches have been developed do deal with changing demands on the building. The most recent development is interactive architecture. In this paper we aim to outline what interactive architecture is. First we define the type of performance behavior that an interactive building or environment has. Following, we consider the relation between the system and the user. We derive four types of relations, characterized as "perfect butler," "partner," "environmental," and "wizard." Interactive systems are composed of sensors, controllers, actuators, and materials. Various degrees of interactivity can be achieved with such systems, ranging from passive, reactive, autonomous, to agent systems. Complete with earlier discussion of design methods this provides the range of aspects that should be considered when designing interactive architecture.