Grid systems are becoming the underlying infrastructure for many high-performance distributed scientific applications. Resources in these systems have heterogeneous features, are connected by potentially unreliable networks, and are often under different administrative management domains. These remarks show how a new generation of techniques, mechanisms and tools need to be considered, that can cope with the complexity of such systems and provide the performances suitable for scientific computing applications. In this paper, an architecture for monitoring services in Grid environments based on mobile agent technology is presented. One of the most interesting aspects of the proposed approach is the possibility of customizing the parameters to be monitored. After describing the general framework where agents will execute, an application for improving the capabilities of traditional network monitoring paradigms will be presented.