In recent years vehicle radar and lidar sensors are widely used as sources of control signals for Autonomous Intelligent Cruise Control and Collision Mitigation systems. These devices are operating in the millimeter wave and infrared range in which their performance may be degraded by adverse weather conditions. Currently available information regarding the signal interaction with fog, rain, snow and spray make clear that millimeter-wave radar sensors are far less affected by adverse weather conditions than infrared-based sensors. However, when automotive radar sensors are particularly designed for safety-oriented systems, the effects of critical issues (such as water film and heavy rains) on the sensor performance become of the uttermost importance. In this thesis, an analysis of the effects of water film and rain on millimeter-wave propagation have been presented. Based on the fundamental formulation of wave propagation and scattering in stratified as well as random media, physical parameters describing the wave interaction with the water film and rains have been studied and assessed. It has been shown that both significantly attenuate millimeter-wave signals, and may exert an adverse influence on detection performance of millimeter-wave radar sensors. Consequently, water-film and rain indicators have been derived from returned signals to convey for the first time wet-antenna and rain information to the radar sensor system, where they could be employed for monitoring the radar detection performance. Methods have also been worked out in detail for low-cost measurement of water-film and rain indicators in the radar system. Experimental investigations have been exhibited to be able to satisfactorily implement these indicators in production. Finally, techniques have been introduced to show how detection performance could be optimized. The presented findings emphasize the importance of identification of water film and rain and of being able to optimize detection performance in order to assure availability of automotive radar senors in adverse weather conditions. Results should also be significant for all kinds of millimeter-wave radar sensors where wave interactions with water film on the surface of the radar antenna or its protecting radome as well as with rains in the radar beams are the key issue.