The present document is an introduction to the TSG RAN TS 36.42x series of UMTS Technical Specifications that define the X2 interface. It is an interface for the interconnection of two E-UTRAN NodeB (eNB) components and an EUTRAN NodeB (eNB) and an E-UTRAN gNodeB (en-gNB) within the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) architecture (TS 36.401 [2]).