We present a experimental study of the analysis of the hydrodynamics evolution of the melt pool during laser Yag welding of stainless steel. The main diagnostic that has been used in this study is a high-speed video camera (10 to 30 kHz) that allowed us to analyze the main different parts that can be observed on such melt pools. For our operating conditions (incident laser power: 4 kW, focal spot diameter: 0.6 mm, stainless steel), we have shown that five characteristic, different and complex hydrodynamic behaviors can be defined when the welding speed varies from typically a few m/min to a few tens of m/min. At low welding speed, the keyhole is quite vertical embedded inside a large pool that fluctuates due to friction effects induced by the quite vertical ejected plume. At high welding speeds, laser interaction is only localized on the keyhole front whose inclination increases with the welding speed. Induced melt flow then dominates and can generate the humping regime, with severe undercuts. For intermediate welding speeds, it is the interaction of the vapor plume with the melt pool that controls its stability. These experiments allow us to confirm that the interaction of the melt pool with the vapor plume, which is emitted with a variable dynamic and direction, perpendicularly from the inclined keyhole front, has an essential role for the melt pool stability and its dynamics in laser welding.We present a experimental study of the analysis of the hydrodynamics evolution of the melt pool during laser Yag welding of stainless steel. The main diagnostic that has been used in this study is a high-speed video camera (10 to 30 kHz) that allowed us to analyze the main different parts that can be observed on such melt pools. For our operating conditions (incident laser power: 4 kW, focal spot diameter: 0.6 mm, stainless steel), we have shown that five characteristic, different and complex hydrodynamic behaviors can be defined when the welding speed varies from typically a few m/min to a few tens of m/min. At low welding speed, the keyhole is quite vertical embedded inside a large pool that fluctuates due to friction effects induced by the quite vertical ejected plume. At high welding speeds, laser interaction is only localized on the keyhole front whose inclination increases with the welding speed. Induced melt flow then dominates and can generate the humping regime, with severe undercuts. For interme...