A very common requirement in RFID systems is the reading of co-located tags, that is necessary when multiple tags are found simultaneously in the coverage area of the tag reader. Due to the low latency and low power constraints of these systems, the availability of efficient protocols able to arbitrate collisions and guarantee shared access to the transmission medium becomes of fundamental importance. This chapter has described several anti-collisions protocols for RFID systems, that can be grouped in the two main categories of deterministic and stochastic protocols. We have seen that deterministic protocols, as the binary tree algorithm, can outperform classic stochastic