The accuracy of Telecom Boundary Clocks (T-BCs) is essential to the successful roll-out of 5G, in addition to supporting LTE-A and TDD-LTE. The ITU-T has updated specifications for T-BCs in Recommendation G.8273.2 to cover this range of applications. This Technical Overview describes what a boundary clock is, and explains the critical performance parameters defined in G.8273.2.