The high reflection and the high thermal conductivity of aluminum make it difficult to transfer the laser energy into the workpiece. For high energy transfer efficiencies a multikilohertz repetition rate laser system with high average power and high peak power is required. By using a Q-switched CO2 laser oscillator amplifier system laser pulses of 70 mJ energy, 0.2 ps half width and 250 kW peak power with a repetition rate of 20 kllz, an average power of 1.4 kW and TEM00 mode quality are available. The pulsed laser system allows the cutting of aluminum plates with a higher velocity and a better quality than it can be reached with a cw laser at comparable average power. Using laser pulses burr free cutting of aluminum up to 10 mm thickness at moderate average laser power level is possible, because a laser-induced plasma is ignited within the gap by each laser pulse. For welding of aluminum plates the laser pulses of the Q-switched laser system can be shaped to achieve an optimized energy transfer into the material and a good welding quality. The electron density of the welding plasma plays an important role for the energy transfer. The spatial and temporal distribution of the electron density in a laser-induced plasma is measured by a beam deflection technique. The electron density can be influenced by the working gas and the laser pulse form. Typical values 1.2 mm above the aluminum surface are for example 3 . 1023 m3 in case of argon as working gas and 1 . 1023 m3 in case of helium.