The laser beam weldability of AZ31B magnesium alloy was examined with high power pulsed Nd:YAG and continuous wave (cw) CO2 lasers. The low solid absorptivity, liquid viscosity, and liquid surface tension of magnesium make it more difficult to weld than steel. Welding parameters necessary to obtain sound autogenous welds were determined for both pulsed Nd:YAG and cw CO2 lasers. The weldability of magnesium was significantly better with the Nd:YAG laser. This observation was attributed to the higher absorption of the Nd:YAG beam, which in turn reduced the threshold irradiance required for welding and produced a more stable weldpool. The signal from an infrared weld monitor was correlated with both the penetration depth and the quantity of black powder on the workpiece after welding. The fine black powder was shown to result from evaporation of magnesium and zinc. The Nd:YAG welds were only slightly softer than the base material and had very narrow heat-affected zones.