The World Wide Web encourages widely-distributed, open, decentralised systems that span multiple administrative domains. Recent research has turned to trust management [4] as a framework for decentralising security decisions in such systems. However, whilst traditional security measures such as cryptography and encryption are well-understood (theoretically and empirically), the same cannot be said for computational trust models. This paper describes the attack-resistance of several well-referenced trust models, in a move toward a possible framework and terminology for such analyses. We present a number of open questions, and consider possible future directions in the area.