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This article describes the timing aspects of optical transport networks as defined by ITU-T SG15 Q13. It first presents the initial decisions that led to the specification of Recommendation G.8251 in 2001, which defined the OTN network limits and the clocks for OTN equipment. This article also presents work done since 2001 to support the evolution of Recommendation G.709, which introduced new OTN mappings such as Ethernet. Among the most significant timing-related aspects are the new mappings that allow timing-transparent transport of synchronous Ethernet.