Applying visual techniques to access data has been a successful way of abstracting and simplifying the complexity of the operation to a user not versed in the technical domain. Visual querying systems have been developed to represent a database schema and to visually query its content. Similar effort has been done for visually representing the knowledge in an ontology. Since ontologies are gaining ground into playing an important role in representing knowledge of multidisciplinary domains, there grows the need for a simple way to query ontologies.  This thesis presents OntoVQL, a formal visual query language that allows to query OWL ontologies. OntoVQL hides the complexity of an OWL query language by abstracting the query as a graph that can be broken and reassembled and is designed to formulate more than simple queries with the and/or constructors. OntoVQI, a visual querying interface is introduced as the prototype that has been implemented to effectively formulate queries in OntoVQL and to view their results. This prototype facilitates query formulation by providing features such as query preview.