Due to the widespread use of radar sensors in the millimeter wave range, multiple sensors are often used in close proximity or in the same application scenario. This can lead to interference between the sensors. In road traffic in particular, many vehicles are nowadays equipped with one or more radar sensors, so that in dense traffic scenarios, interference can occur between sensors on different vehicles. This chapter first presents an overview on interfered signals in radar applications and then provides a more specific description of the impact of interference on analog and digital radars. Additionally, a selection of countermeasures is presented.