Radio-frequency identification (RFID) is an up-and-coming technology. The major limitations of RFID technology are security and privacy concerns. Many methods, including encryption, authentication and hardware techniques, have been presented to overcome security and privacy problems. This paper focuses on authentication protocols. The combination of RFID technology being popular but unsecure has led to an influx of mutual authentication protocols. Authentication protocols are classified as being fully fledged, simple, lightweight or ultra-lightweight. Since 2002, much important research and many protocols have been presented, with some of the protocols requiring further development. The present paper reviews in detail recently proposed RFID mutual authentication protocols, according to the classes of the authentication protocols. The protocols were compared mainly in terms of security, the technique that they are based on, protocols that the presented protocol has been compared with, and finally, the method of verifying the protocol. Important points of the comparisons were collected in two tables.