Underpinning every digital library and digital repository there is a policy framework, which makes the digital library viable - without a policy framework a digital library is little more than a container for content. Policy governs how a digital library is instantiated and run. It is therefore a meta-domain which is situated both outside the digital library and any technologies used to deliver it, and within the digital library itself. Policy is also a key aspect of digital library and digital repository interoperability in a common and integrated information space. Policy interoperability - that is the exchange and reuse of policies - is a step beyond policy standardisation. Furthermore, effective and efficient policy frameworks are also one of the Digital Curation Center (DCC), DigitalPreservationEurope (DPE), nestor and Center for Research Libraries (CRL) core criteria for digital repositories. In this article, we share our research on policy interoperability levels and the experimental survey on policy interoperability conducted with real-life digital libraries, as a contribution towards the definition of a Policy Interoperability Framework.