This paper presents the support for user-defined metrics in the G-PM performance analysis tool. G-PM addresses the demand for aggressive optimisation of Grid applications by using a new approach to performance monitoring. The tool provides developers, integrators, and end users with the ability to analyse the performance characteristics of an application at a high level of abstraction. In particular, it allows to relate an application’s performance to the Grid’s performance, and also supports application-specific metrics. This is achieved by introducing a language for the specification of performance metrics (PMSL) and the concept of probes for providing application specific events and data. PMSL enables an easy specification of performance metrics, yet allowing an efficient implementation, required for the reduction of monitoring overhead.