This paper presents the design, fabrication, and performance of a 77 Ghz monolithic GaAs PIN switch network developed for an automotive collision warning radar. The RF front end of the radar contains three control circuits which were initially realized by a hybrid approach using a duroid circuit and beam lead diodes. These three circuits were successfully integrated into a single 77 Ghz MMIC. The MMIC uses vertical GaAs PIN diodes with a switching cutoff frequency of over 3000 Ghz, attaining low insertion loss and high isolation. Insertion loss is comparable to that obtained with a hybrid circuit, while the isolation improved from a typical level of 16 dB for a single hybrid switch to over 25 dB in the MMIC. The use of GaAs PIN diodes also allowed switching speeds of less than 2ns to be attained, a key requirement for the radar.being realized by a multichip approach, where the best available device technology is used for a specific circuit function. The switching functions required by the pulsed Doppler radar described in this paper are best realized by a monolithic GaAs PIN diode approach. Radar Architecture MMIC SWlTCH CONTROLS