This Handbook provides an overview of the 18 ITU-T Recommendations dealing with the xDSL technology published as of the end of 2009. After a brief introduction of the xDSL technology, the xDSL Recommendations are grouped into nine categories: HDSL and SHDSL, five ADSL (G.992.2, splitterless ADSL, ADSL2 reach extended, splitterless ADSL2 and ADSL2plus) and two VDSL (VDSL1 and VDSL2). Operations and signalling principles of these nine families of xDSL systems are presented, emphasizing the commonalities.