Transacting and interacting through computer networks makes it difficult to use traditional methods for establishing trust between parties. Creating substitutes by which people, organisations and software agents can derive trust in others through computer networks requires computerised analysis of trust topologies. This paper describes diverse dimensions of trust that are needed for analysing trust topologies, and provides a notation with which to express trust relationships in terms of these dimensions. The result is a simple way of specifying topologies of trust from which derived trust relationships can be automatically and securely computed.