Lincoln Laboratory, under FAA sponsorship, is developing an airborne collision avoidance system (TCAS II), concentrating primarily on the air-to-air surveillance subsystem. The surveillance functions required are to detect the presence of nearby transponder equipped aircraft, and then generate a surveillance track on each aircraft, issuing range and altitude reports once per second. The development effort from mid-1981 to the present has focused on the surveillance problems associated with high aircraft density. A number of surveillance techniques to deal with the high density environment have been identified and evaluated mainly through airborne measurements. A TCAS II design was synthesized, and this design was subjected to in-flight testing in the Los Angeles Basin using a Boeing 727. Results indicate that the performance objectives have been met.