Policy languages have become the most common way to protect the online resources from unauthorized access. Nowadays, the Sun's XACML has become the most widely used standard for access control, allowing the definition of hierarchical policies and very complex rules. From the idea that semantic technologies could help the definition and execution of access control policies, in this thesis we want to analyze the state of the art of the emerging semantic web technologies, focusing on the semantic approaches to access control, and in case there are any, the approaches whose base stands on XACML.