The replication licensing system acts to ensure that no section of the genome is replicated more than once in a single cell cycle. Experiments using Xenopus egg extracts have revealed that the licensing system consists of two components, named RLF-M and RLF-B. Whereas the function of RLF-B is still unclear, RLF-M has been shown to consist of all six members of the MCM/P1 family proteins, which appear to be the structural component of the licensing system. The origin recognition complex (ORC) and Cdc6/Cdc18 are needed on chromatin before the licensing reaction can take place, although they are not themselves components of the licensing system. Cell cycle events and cyclin-dependent protein kinases (Cdks) also seem to be involved in controlling the licensing system to ensure once per cell cycle DNA replication. The subject of this review is to detail our current understanding of the licensing system and the way that it interacts with other components of the cell cycle machinery.