Laser-arc hybrid welding has developed into a viable industrial technology in recent years with a number of technological applications. The physics of the underlying interactions between the laser beam and arc plasma is quite complex and, in order to explore the relationships involved, it is useful first to consider important aspects of arc and laser welding separately. The physics of laser welding has already been examined in Chaps. 4 and 5. A generic description of welding arcs is therefore provided here, which forms a basis for interpretation of laser-arc interactions and the hybrid welding conditions discussed in the final section of this chapter.