In RFID systems, the primary aim of the reader is to identify tags quickly. Hence, anti-collision protocols play an important role in arbitrating tag replies such that a reader experiences minimal or no collisions in each read round. Moreover, they must minimize idle slots to ensure high system efficiency. Otherwise, a RFID reader will experience prolonged identification delays, and also energy and bandwidth wastage. Henceforth, this chapter presents a comprehensive review of Aloha based anti-collision protocols. Specifically, we will present Pure Aloha (PA), Slotted Aloha (SA), Framed Slotted Aloha (FSA), and theirvariants. In addition, we will analyze their read performance with increasing number of tags.