Distributed systems based on cluster of workstation are more and more difficult to manage due to the increasing number of processors involved, and the complexity of associated applications. Such systems need efficient and flexible monitoring mechanisms to fulfil administration services requirements. In this paper, we present PHOENIX, a distributed platform supporting both applications and operating system monitoring with a variable granularity. The granularity is defined using logical expressions to specify complex monitoring conditions. These conditions can be dynamically modified during the application execution. Observation techniques, based on an automatic probe insertion combined with a system agent, allow the system to minimize the execution time overhead. The platform extensibility offers a suitable environment to design distributed value added services (performance monitoring, load balancing, accounting, cluster management, etc.).