Providing applications with semantics is an upward trend, so it may also seem interesting to add semantic technologies to the security and trust field, because of its multiple benefits. Semantic technologies allow us to improve systems' interoperability and integration and also permit the use of inference techniques. In this thesis we will analyze the state of the art around semantic technologies and already developed ontologies in the security and trust field. On the basis of this study, we will develop a proposal on security and trust to define which concepts need to be modeled, giving more importance to the PKI field which is our main interest, developing the needed ontologies in case some of the concepts are not modeled yet by any ontology.