The paper investigates the consensus problem in anonymous, failures prone and asynchronous message passing systems. It determines the weakest failure detector in anonymous message passing systems for solving consensus. Two failure detector classes AΣ' and AL are introduced, which may be seen as the anonymous counterparts of Σ and Ω, the weakest failure detectors for consensus in non-anonymous system. An anonymous consensus algorithm that relies on AΣ' and AL is presented. The paper then establishes that AΣ and AL can be distributively emulated given any failure detector D that can be used to solve consensus, thereby proving the necessity of AΣ' and AL to solve crash-tolerant consensus in anonymous systems.