The present document describes the implementation of a Lawful Interception interface in a TETRA system. It provides the requirements and specification of the interface within a TETRA system for the purpose of providing data to Law Enforcement Agencies (LEAs) in the area of Lawful Interception (LI) of communications. The provision of a Lawful Interception interface for TETRA is a national option@ however where it is provided it shall be provided as described in the present document. The structure of lawful interception in telecommunications is in two parts: The internal interface of a network that is built using a particular technology; and@ the external interface (known as the Handover Interface) that links the LEA to the network. Between these two parts may lie a mediation function to cater for national variances and delivery of the result of interception. The Handover Interface may be the subject of national regulation and therefore the mediation function may be a matter of national regulation. The subject of the present document is the internal LI interface that lies between the TETRA infrastructure and the mediation function. The present document describes the data content of information flows from the TETRA system to the mediation function. It does not describe a communications protocol stack but assumes the use of one with entry made at layer 7 (application layer). The EN has been written with ROSE as a target layer 7 protocol and with the ASN.1 Basic Encoding Rules (BER) as the target layer 6 (presentation) protocol. To facilitate this the data definitions are made with ASN.1. This method allows configuration of either local or remote mediation functions. The EN does not specify how ROSE and BER are used. The present document is structured as follows: ?clause 4 outlines the essential requirements for the TETRA LI interface; ?clause 5 presents the structural and behavioural models of the LI interface; ?clause 6 presents the data model and allocation behaviour in the LI interface. The present document applies to TETRA services where access to the communication of TETRA Subscriber Identities (TSIs) is available in a network (Switching and Management Infrastructure (SwMI) or Radio Packet Data Infrastructure (RPDI)). Whilst this does not prohibit lawful interception of TETRA Direct Mode Operation (DMO) it removes the liability of network operators and service providers to provide a result of interception when communication does not make use of their networks. The present document describes the normal and exceptional operation in each of the three operational phases of T-LI: 1 Setup The actions taken within the TETRA network to establish the monitoring of a target and the communications paths to the mediation function. 2 Monitoring The monitoring of target activity and its delivery to the mediation function. 3 Cleardown The removal of a monitor facility against a target and the cleardown of the communications paths to the mediation function. The present document does not describe the means of transporting data from the TETRA network to the LEA@ but describes only the means of capturing and encoding the activities of a target within the TETRA network and delivering this data to the mediation function. The present document does not define the operations or technical requirements of the Handover Interface that takes data from the mediation function to the LEMF. The present document does not define the operations or technical requirements of the Law Enforcement Monitoring Facility (LEMF). NOTE 1: The present document presupposes some familiarity with the operation of TETRA systems and of lawful interception. NOTE 2: The present document suggests a barrier to external manipulation of the TETRA infrastructure by means of a mediation function. NOTE 3: No testpoint is provided in the present document to ensure conformance. This is addressed national standards pending the completion of a common handover interface being developed by ETSI TC SEC-LI under work item DES/SEC-003003 and to which the present document is provided as input.