Attenuation measurement is an important part of characterising radio frequency (RF) or microwave circuits and devices. There are manufactured devices with well-defined losses, called attenuators, used for measurement and for adjusting power levels to a certain desired value. Attenuators are probably the most important devices in measurement systems and therefore exist in a large number of different forms, such as symmetric, coaxial, waveguide, optical, fixed-value, and variable-loss. For this reason, it is growing the necessity to measure the component's loss as accurately as possible to optimize system parameters. In the following book is described the system developed at Van Swinden Laboratory (VSL), the Netherlands' national metrology institute (NMI), to perform high-accuracy attenuation measurement up to 50 GHz. One of the key inventions in this work is the introduction of the VNA as part of the system. The results show the potentiality of the proposed method. This approach provides much higher flexibility, functionality, performance and speed.