A nonintrusive, solid-state device has been developed to monitor in real time the infrared emissions during laser welding. The weld monitor output is an analog signal (100–1000 mV) that depends on the beam power and weld characteristics. The dc level of this signal is related to weld penetration, while ac portions of the output can be correlated with surface irregularities and part misalignment or contamination. Changes in dc behavior are also noted for both full and deep penetration welds. Full penetration welds are signified by an abrupt reduction in the weld monitor output. Bead-on-plate welds were made on steel, aluminum, and magnesium with both a cw CO2 laser and a pulsed Nd:YAG laser to explore the relationships between the weld characteristics and the weld monitor output.