Abstract: Applications such those of electronic payment require the participants authentication and a privacy of crucial information. Public-Key Infrastructures (PKIs) are essential for providing them these security services in open networks like Internet. We propose in this paper a logic for reasoning about PKI trust models. Our formalism enables us to describe a trust model of a PKI with greater precision than the widely used graph. It also allows us to verify whether a model respond to the PKI objectives and under which conditions. These objectives generally concern statements about entities beliefs with regard to public key authenticity and CAs trustworthiness. The proposed formalism takes into account the number of intermediates that have participated in an entity belief and the constraints that can be the concern of certification practices or certificate policies of PKI's CAs. These constraints may influence the trust model suitability to applications with specific requirements.