This book was written in an attempt to present the state of the art in the utilization of the ontologies in the public administration. The term ontology comes from philosophy. Translation from Greek would be: Studying the nature of existence. In computer science, the most commonly quoted definition of ontology is given by the Gruber: Ontology is an explicit conceptualization specification. One of the interpretations of this definition would be that conceptualization refers to abstract a model of a phenomenon in the world, identifying the relevant concepts of this phenomenon. Explicit means that an explicitly defined type of concept is explicitly defined used as well as the limitations of their application. Typically, ontology consists of the list of terms and relationships between these terms. The terms designate concepts (classes or objects) of a domain. Relationships usually consist of a hierarchy of class.