In many modern radar applications in the millimeter wave range, the radar must determine not only the range and velocity of a target, but also its angle at which it is located in relation to the radar. For this purpose, antenna arrays are required, i.e. a group of antennas. By evaluating and comparing the signals at different receiving antennas, the angle of incidence, i.e. the angle of the target, is determined. The larger an antenna array is in relation to the wavelength, the better its angular resolution and separation capability. For this reason, the millimeter wave range is interesting for angle estimation due to its small wavelengths. The absolute size of the sensors remains in the range of a few centimeters for most applications, even with high demands on angular resolution. While in the past, radars often employed mechanically rotating antennas with strong beam focusing to determine the angle of incidence, nowadays only electronically scanning systems are used in the millimeter wave range. Thus, a mechanical scanning system for the antenna is no longer required. The electronic approaches, known as beamforming, can be realized in both analog and digital domains. In this chapter antenna arrays are described both from the point of view of an antenna engineer and from the point of view of signal processing to show the duality and differences of both approaches. Additionally, hardware influences on array signal processing and calibration are discussed and the concept of a beamformer is presented. Finally, the achievable resolution and separability of one- and two-dimensional arrays are derived.