Under the Third Energy Package Directives 2009/72/EC and 2009/73/EC, European energy networks are subject to unbundling requirements which oblige Member States to ensure the separation of vertically integrated energy companies, resulting in division of the various stages of energy supply (generation, distribution, transmission and supply). This paper addresses the challenges that a regulatory compliance officer faces in line with the Third Package provisions. The focus is on the application of the rules on the unbundling independent transmission operator (ITO) model. The present document highlights the Compliance Officer's practice in dealing with certain aspects of the rules on unbundling of Transmission System Operators (TSOs), as laid down in the Gas Directives, such as developing an Effective Compliance Monitoring Program, and setting out practical rules to be observed by staff in relation to non-discrimination, transparency and the handling of confidential information. As a case study, this experience from the Greek Gas TSO, i.e. the Hellenic Gas Transmission System Operator (DESFA), is presented.