This paper presents a new monitoring principle for laser transmission welding of plastics. The principle uses two independent detectors: a CCD or CMOS camera to supply an image of the weld seam and a pyrometer to detect heat radiation from the welding process. In laser transmission welding, the weld seam is usually hidden by the laser transparent joining partner, which may still be colored in the visible spectrum. Therefore, a special lighting laser will be used to acquire an image of the weld for CCD or CMOS cameras. If the upper joining partner is not only partly transparent for the laser wavelength of the welding laser, but also for the laser wavelength of the lighting laser, a direct visualization of the weld is possible. Factors influencing the weld quality or leading to defects within the weld are kerfs on the surfaces of the joining partners, moisture uptake of the plastic material and the surface finish of the materials. Which weld defects result from these factors and how they can be detected by the monitoring system are shown. Not all of the resulting defects can be detected with a single detector. Therefore, the information from the two detectors, camera and pyrometer, is correlated.