Peers in Structured Peer-to-Peer Systems (SP2P) act autonomously: They are able to choose their contacts as well as their strategy, i.e. how to interact with their contacts. Participants of SP2P Systems are able to use free-riding on both levels. More precise, they can drop queries on behalf of other peers, or they can avoid contributing to the network structure, i.e. can have less contacts than anticipated by system designers. Our objective is to understand how peers in SP2P Systems choose their contacts as well as their strategy (concerning the queries of their contacts) in networks that change dynamically. If it turns out that free-riding is dominant in such systems, we want to design mechanisms that foster cooperation, or that enhance network efficiency, respectively.