Radar sensor technology has spread immensely over the past decade and is now indispensable in many applications. This is mainly due to the fact that the costs of radar sensors have fallen tremendously and radar sensors have strengths in areas that are not covered by many other sensor principles. For example, radar is extremely robust against environmental influences such as weather, lighting, the position of the sun, dirt or other environmental influences in which the radar sensors are used. In addition, radar is one of very few sensor principles that allows direct velocity acquisition. Radar is an active sensor principle: A radar sensor transmits an electromagnetic wave that is reflected by a target in the radar channel and received back by the radar sensor. By comparing the received signal with the transmitted signal, various information about the target can be obtained. The most important properties are the range of the target to the radar sensor, the radial velocity of the target in relation to the radar sensor, and the direction in which the sensor detects the target. The comparison of the received signal with the transmitted signal contributes to the robustness of the system, as interference or signals emitted by other electronic devices are not mistakenly interpreted as part of the received signal. In this chapter, the fields of application frequently used and discussed today and the corresponding frequency ranges are presented. This is followed by the basics of radar sensor technology with a derivation of the fundamental radar equation and an introduction to key terms and measurable variables. The chapter concludes with a brief insight into wave propagation in the millimeter wave range, which includes the influence of the atmosphere and multipath propagation.