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There are various routes to system validation for helicopter health and usage monitoring systems (HUMS). Up-front validation through extensive test and qualification procedures is one approach, popular with aircraft operators seeking to extract maintenance credits from reluctant Design Authorities (DAs), but this is a costly and time-consuming avenue to take. An alternative system validation option is through accumulation of in-service experience and verification by demonstrated reliability. This approach is suitable for operators seeking to realise operational benefits such as predictive maintenance and increased operational availability, but may never win full support from the aircraft DA. The United Kingdom Ministry of Defence (UKMOD) is engaged in several simultaneous HUMS introduction programmes, involving examples of both validation methodologies. An AgustaWestland HUMS capability is being introduced across the EH101 Merlin fleets with extensively tested and qualified software, fully integrated with the flight control systems and flight-critical sensors and implemented through the 'glass cockpit' modular avionics. This system has demonstrated strengths and weaknesses fundamental to the design ethos. In parallel, a Smiths aerospace generic HUMS capability is being retrofitted to the fleets of four helicopter platforms - Chinook, Sea King, Puma and Lynx. This monitoring system is qualified and tested to a lower software integrity level than the Merlin system, has a stand-alone pilot interface and is isolated from flight-critical systems and sensors. In the four years since fielding, this programme has encountered a number of significant successes and obstacles. The Assistant Directorate of aircraft integrity monitoring (AD AIM), a specialist support unit within the corporate UKMOD is in the unique position of having full visibility of both systems. This paper describes the steps taken, the lessons learned and the achievements made in validating and verifying HUMS from a user's perspective.