When laser welding plastics, a distinction can be made in principle between butt welding and transmission welding according to the joint geometry. In industrial applications, mainly transmission welding is used. Here, two plastics with different absorption properties, which are caused by different filler types or concentrations, are welded in a lap joint. The plastic parts to be joined lie on top of each other and the laser radiation is focused through the plastic, which is transparent for the laser wavelength, onto the absorbing moulded part, causing it to melt on the surface. The transmissive partner is also plasticized via heat conduction, so that a connection is formed. In addition to the joint geometry, the type of energy input in laser plastic welding is an important feature for the classification of the processes. Basically, a distinction is made between contour, simultaneous and quasi-simultaneous welding. Since each of the processes has its specific advantages and features, it must be decided for each application case which type of energy input is to be used.