In the last few years, the Semantic Web gained scientific acceptance as the means of sharing knowledge in different domains, and the cornerstone of the Semantic Web is ontologies. Currently, users trying to incorporate ontologies in their applications have to rely on their experience to try to find a suitable ontology for their applications. Methods for evaluating ontology quality and validity, ontology characterization and ranking have been developed for that purpose. In this chapter, we introduce several approaches that have been developed to aid in evaluating ontologies. In addition, we present highlights of OntoQA, an ontology evaluation and analysis tool that uses a set of metrics measuring different aspects of the ontology schema and knowledgebase to give an insight to the overall characteristics of the ontology. It is important to keep in mind while reading this chapter that the definition “goodness” or the “validity” of an ontology might vary between different users or different domains.