Deeply penetrated welds can be produced with high-power-density lasers. Thus the factors affecting laser weld penetration,laser-induced plume behavior and the interaction between a laser beam and a plume,keyhole behavior,melt flows in the molten pool,and the generation and prevention of bubbles and porosity were investigated. Consequently,the effects of plasma and plume on the weld penetration were interpreted in CO₂ and fiber lasers welding,respectively. The formation of spattering,underfilled beads,and humping was understood. It was confirmed that porosity was easily formed at high powers and low welding speeds due to the generation of many bubbles from the tip of a keyhole.