This article investigates privacy in Radio Frequency IDentification (RFID) systems. We survey the eight most well-known RFID privacy models. We examine their advantages and drawbacks, and provide a comprehensive comparison of these models. The first conclusion is that none of these models is complete, and the association of all their positive features does not either provide the best privacy model. Then, we give a privacy analysis of five distinct RFID authentication protocols according to these models, and exhibit a main worry: no model is able to distinguish perfectly all the five RFID protocols regarding privacy.