Recommendation ITU T Z.164 defines the operational semantics of Testing and Test Control Notation 3 (TTCN-3). The operational semantics are necessary to unambiguously interpret the specifications made with TTCN-3. This Recommendation is based on the TTCN-3 core language defined in Recommendation ITU T Z.161. This revision of the Recommendation contains amendments, clarifications, corrigenda and editorial corrections. This Recommendation is technically aligned with ETSI ES 201 873-4 V4.6.1 (2016).