An effective GSS is vital to the successful implementation of an EMS and is a fundamental part of the total monitoring system design, including asset management. Unlike the on-board part of the EMS which principally uses real time data to indicate when engine maintenance is required, a GSS can offer much greater processing power to comprehensively analyze and manipulate EMS data for both maintenance and logistics purposes. This document reviews the main EMS functions and discusses the operating requirements used to determine the basis design of a GSS, including the interfaces with other maintenance or logistic systems. A brief discussion is also included on some of the more recent advances in GSS technology that have been specifically developed to provide more effective diagnostic capabilities for gas turbine engines.