Communications security is a central aspect of the ISITEP framework for inter-system interoperability between TETRA and TETRAPOL networks. Ensuring that threats to the interconnected communications systems and terminals are sufficiently and appropriately reduced by technical, procedural and environmental countermeasures is vital to realise the trusted and secure communication system needed for the pursued PPDR transnational cooperation activities. In this context, ISITEP Work Package 2.2 (WP2.2) “Security Requirements” is aimed at establishing the set of security requirements needed to drive the development of the security architecture and its components. This first deliverable from WP2.2 provides a description of the preliminary framework and methodology defined to carry out the development of the security requirements. The framework includes aspects such as the identification of the ISITEP system components and players that are relevant for the security analysis, and the definition of security objectives. On this basis, the document also conducts a first assessment to delineate the state-of-the-art of the security features existing in current PPDR technologies and networks, security procedures carried out by PPDR end-users and operators in order to realise the trusted and secure communication system needed in national PPDR networks, and to provide an overview of the regulatory and legislation framework that impacts affects PPDR communication networks. Finally, a preliminary assessment of security threats to ISITEP system is also reported. The assessment is focused on those security threats that are relevant to the new communications capabilities brought by the ISITEP solution in terms of service interworking across multiple national networks and terminal roaming. ; Entregable del projecte de recerca ISITEP ; Preprint