A dense radio frequency identification network, consists of several readers located in close proximity to improve coverage and read rate. In such environment, reader to reader collision (RRC) is a common and inevitable problem. Although a number of algorithms has been proposed to mitigate these collisions, most of them require extra hardware, do not make an effective use of the network resources or they are incompatible with the current standards and regulations. This paper reviews the significant proposals for anti-collision of RFID readers, coordination in passive and dense reader environment (DRE). The short comings and incompatibilities with the state of the art standards like EPCglobal Class-1 Gen-2 are also discussed. Finally, research directions to improve the current RRC approaches are presented. © 2013 IEEE.