ABSTRACTAlthough rig testing is widely used in advanced research, testing of complete marine gas turbine engines (under controlled environmental conditions) is the economical method for determining specific suitability and weak areas. The damaging effects of salt water introduced by working fluids combined with long periods of high power operation predominate in the marine field. Naval gas turbines also endure frequent starts and thermal cycling. Although the hot section (high temperature nozzles, turbine blades and combustors) is most vulnerable and justifiably the focus of testing attention, deterioration of moderate temperature parts (power turbine, compressor, casing, support structure) must be alertly evaluated. The test agenda is designed to explore the engine's ability to meet expected service commitments. Inspections – visual, dimensional, photographic, chemical and metallurgical – provide facts needed for engine evaluation and improvement. The importance of diagnostic testing is defined, discussed and illustrated.