This paper provides information on site-associated phenomena that affect the proper operation of a Mode S sensor and therefore warrant serious consideration when siting a sensor. The Mode S related discussion is intended to be a supplement to the ATCRBS siting criteria presented in the FAA Primary/Secondary Terminal Radar Siting Handbook. The paper discusses siting criteria as they relate to the Mode S sensor antenna system, as opposed to the ATCRBS hogtrough antenna, and importantly, addresses those characteristics of the surrounding environment that are crucial to proper Mode S surveillance.