A PKI can be described as a set of technologies, procedures and policies for propagating trust from where it initially exists to where it is needed for authentication in online environments. How the trust propagation takes place under a specific PKI depends on the PKI’s syntactic trust structure which is commonly known as a trust model. However, trust is primarily a semantic concept that cannot be expressed in syntactic terms alone. In order to define meaningful trust models for PKIs it is also necessary to consider the semantic assumptions and human cognition of trust relationships, as explicitly or implicitly expressed by certification policies, legal contractual agreements between participants in a PKI, and by how identity information is displayed and represented.