This book presents an investigation of the initial moment of formation and propagation of hydrogen cracking in welded joints of an API 5L X80 steel, using the Implant and Tekken tests coupled with the acoustic emission (AE) technique. The development of a System of Acoustic Emission - SEA and a computational program to register the acoustic signals generated by the crack events is presented. In the development of the work two welding processes were used (Tubular Wire and MIG/MAG) with consumables of chemical compositions and different mechanical properties of the base metal. The protection gases were mixtures Ar+25%CO2 and Ar+25%CO2+3%H2, the latter to induce high values of diffusible hydrogen in the joint as well as the use of two welding energy levels, 1,058 kJ/mm and 1,500 kJ/mm.