The present document defines the Terrestrial Trunked Radio system (TETRA) Direct Mode of operation. It specifies the basic Air Interface (AI)@ the interworking between Direct Mode Groups via Repeaters and interworking with the TETRA Trunked system via Gateways. It also specifies the security aspects in TETRA Direct Mode and the intrinsic services that are supported in addition to the basic bearer and teleservices. This part describes the security mechanisms in TETRA Direct Mode. It provides mechanisms for confidentiality of control signalling and user speech and data at the AI. It also provided some implicit authentication as a member of a group by knowledge of a shared secret encryption key. The use of AI encryption gives both confidentiality protection against eavesdropping@ and some implicit authentication.