The aim of this chapter is to review our main recent understanding of physical mechanisms occurring during keyhole laser welding. The focus is on the analysis of melt pool dynamics showing that it is the interaction of the vapour plume emitted from the keyhole front with the melt pool that plays a dominant role in melt pool dynamics. Different specific regimes concerning the behaviour of the melt pool for a large range of welding speeds can be observed, and these regimes are precisely defined by the inclination of the keyhole front. a model taking into account these physical processes is proposed and allows us to describe the keyhole parameters and to understand these different observed results
