Metrology is the art and science of measurement. Achieving precise and reliable measurements, including a quantification of their accuracy, has been the goal for the past centuries. Equipment, methods, and tools have been developed in many disciplines of science and technology to realize today’s state of the art. In many cases, everyday life relies on trust in measurements. Classical examples are fair exchange of trade goods and pricing, proof of legal requirements such as compliance with safety limits, and comparability of scientific results but also technological specifications that are required for interoperability. Although much of the communication infrastructure is data-driven and defined by specification standards, metrology and traceability are essential to build and maintain the system. Also, with a growing number of applications making use of terahertz radiation, the question of the reliability of measurements in the terahertz frequency range becomes increasingly important. In this chapter we give a general introduction into metrology and its application to communication systems.