Adaptive web sites are sites that automatically improve their internal organization and/or presentation by observing userbrowsing behavior. In this paper we argue that adaptive behavior of websites should be controlled in order to keep the website manageable. We believe that adaptive behavior may be a useful complement to a good website design method on the condition that the adaptations are limited and according to the modeling approach followed during design. Therefore, we allow a website designer to specify at design time the adaptive behavior that will be allowed at runtime. To accomplish this goal, an Adaptation Specification Language is defined that allows designers to specify at the level of the navigational model, which adaptations could be performed at runtime. The language is event based, i.e. a collection of rules is used to specify the adaptation operations to be carried out if certain conditions are satisfied. The approach proposed is elaborated in the context of WSDM, an...