ITU-T Recommendation K.27 provides the procedures for implementing the bonding between metallic bodies and the earthing connections inside a telecommunication building. It provides the bonding configurations and earthing in order to achieve protection against electric shock and to minimize damage of the telecommunication equipment due to lightning flashes, as well as to limit interference between telecommunication equipment installed in the same building. Are examples of the metallic bodies considered in this Recommendation: equipment frame, shield of cables, metallic structure of the building, protective earth conductor, bonding conductors, earthing electrodes, etc. This Recommendation provides different possibilities of bonding configurations and discusses its advantages and disadvantages. The theory of bonding and earthing is also presented and it is used as rationale for the development of the procedures.