Abstract: Monitoring distributed applications executed on a computational Grid is challenging since they are executed on several heterogeneous nodes belonging to different administrative domains. An application monitoring service should provide users and administrators with useful and dependable data on the applications executed on the Grid. We present in this paper a grid application monitoring service designed to handle high availability and scalability issues. The service supplies information on application state, on failures and on resource consumption. A set of monitoring mechanisms are used according to grid node nature to effectively monitor the applications. experiments on the Grid'5000 testbed show that the service provides dependable information with a minimal cost on Grid performances.