Current formal models of trust are limited since they only consider an agent’s trust in the atomic action of another agent and therefore do not apply to trust in complex actions where the elements in the complex action are atomic actions of dierent agents. Our aim is to present a logical formalization of trust in complex actions, and to show that this formalization can be useful for the formal characterization of trust in composite services, where trust in a composed service is defined in a compositional way from trust in the components of that service.