In recent years, the design and development of military propulsion systems have greatly benefited from advancements in analytical modeling and prediction tools. However, flight testing continues to play an integral role in the maturation of fighter engines. The severity of the fighter mission role dictates the need for capabilities beyond those normally attributed to commercial propulsion systems. While comprehensive static and simulated in-flight engine testing are key elements in developing these capabilities, flight testing provides the avenue for verification that all engine and control functions operate as intended in an integrated environment and that the propulsion system meets with customer, and specifically, pilot approval. In addition to the role that flight testing plays in introducing new propulsion technology into operational service, an equally critical function is served by providing the opportunity for component improvement verification. The USAF CIP program,, aimed at establishing and satisfying the requirement for continuous improvement of propulsion system capabilities, has resulted in numerous upgrades to fighter engines such as the Pratt & Whitney F100-PW-229 for the McDonnell-Douglas F-15E Strike Eagle and the Lockheed-Martin F-16C/D Fighting Falcon aircraft. At present, more than 300 PW-229 engines are installed in USAF and foreign military service (FMS) F-15E and F-16 aircraft and have accumulated more than 310,000 flight hours since operational introduction in 1991. In general, engine improvements center on the areas of durability/maintainability and operability. Using the PW-229 Increased Performance Engine as its example, this paper will focus on various enhancements incorporated since Initial Service Release Program completion in 1991 and the crucial role that flight test has served in ensuring timely maturation of this world-class fighter engine.