Terahertz (THz) spectroscopy holds a high potential as non-destructive, contact-free testing tool for the analysis of macromolecules, the monitoring of plastic processing and the inspection of plastic components. Even molecular properties, such as the glass transition temperature of highly-crystalline polymers, can be derived from THz measurements. Furthermore, we have identified a plethora of emerging applications in the plastics industry. To appeal to these upcoming challenges, we developed the first THz time-domain spectroscopy (TDS) system for the inline monitoring of compounding processes, which was awarded the Otto von Guericke Prize 2009. It has been demonstrated that the filler content at the end of the extrusion line could be precisely determined by time-of-flight measurements. In addition, THz technology is capable of measuring the water content sensitively, THz birefringence measurements reveal the fiber orientation in reinforced plastics and the quality of plastic weld joints or adhesive bonds can be inspected by the interference evaluation of the THz data. Due to the outstanding dielectric contrast of materials at THz frequencies, even contaminations invisible to x-rays or ultrasonic measurements can be revealed by THz TDS imaging opening the door for a new generation of non-destructive, contact-free quality control systems.