During laser beam welding, a laser induced metal vapour plasma is formed. The plasma shows strong fluctuations combined with sound emission. The origin of these fluctuations is discussed. On-line control of the welding process is possible by measuring and analyzing the plasma emission with a photodiode and the sound emission with a microphone. Optical diagnostic methods are compared with acoustical methods. The results show that both diagnostic methods yield complementary information on the process. On-line control of the welding process is demonstrated for the influence of the process gas, hole formation in the seam and welding through.