This paper describes a technique that enables TCAS II to use passive surveillance data obtained via extended squitter, an implementation of automatic dependent surveillance broadcast (ADS-B). The technique, known as hybrid surveillance, is based upon the use of TCAS active surveillance to perform validation of the reported ADS-B position at track acquisition. Aircraft that pass initial validation are maintained on passive surveillance until they become a near threat. At that time, TCAS begins regular active surveillance and thus uses its current surveillance techniques for traffic and resolution advisories. In this way, TCAS is able to use passive extended squitter data while retaining its role as an independent monitor. Simulation results show that the use of passive information for non-threatening aircraft results in a significant decrease in TCAS interrogation rate. This enables TCAS to delay or avoid the range reduction that is now required in order for TCAS to remain within its interference budget in high traffic density airspace. Maintaining TCAS operating range in high density air-space enhances TCAS ability to support situational awareness for the flight crew.