Abstract. Between high-performance clusters and grids appears an intermediate infrastructure called cluster grid that corresponds to the interconnection of clusters through the Internet. Cluster grids are not only dedicated to specific applications but should allow the users to execute programs of different natures. This kind of architecture also imposes additional constraints as the geographic extension raises availability and security issues. In this context, authentication is one of the key stone by providing access to the resources. Grid5000 is a french project based on a cluster grid topology. This article expounds and justifies the authentication system used in Grid5000. We first show the limits of classical approaches that are local files and NIS in such configurations. We then propose a scalable alternative based on the LDAP protocol allowing to meet the needs of cluster grids, either in terms of availability, security and performances. Finally, among the various applicati...