RF components are created so that they can be used in systems. The design of RF systems is a huge field, ranging from scalar measurement technology to highly complex mobile radio systems. This chapter introduces the basic techniques and important parameters of simple transmission systems. To understand common measurement and communication systems, it is essential to introduce other important RF components. Components such as detectors, mixers, IQ modulators, oscillators, synthesis generators and antennas are introduced. From the large number of possible systems, the scalar and vectorial S-parameter measurement systems will be presented as examples. These are very clear and form the basis for many more complex RF systems. Furthermore, many measurements of physical quantities and effects such as humidity, dust, temperature, lengths, distances, speeds and more can be carried out using S-parameter measurements. The basics for the transmission of so-called IQ signals are then presented. Finally, a new innovative and easy-to-implement radio system based on the use of two modes is presented as a system solution.