M. Hofmann Proxy services are a new class of applications running on caching proxies or dedicated application servers, preferably at the network edge. They are described in [2] and [3]. Execution of proxy services is triggered by certain conditions. These conditions are service specific and have to be provided by the party on behalf of which the affected service modules are executed. The Proxy Service Rule Specification Language (PSRL) is an XML-based language that can be used to describe service specific execution rules. It allows a service provider to tell a proxy caching provider when and how the services should be executed.