This paper discusses the concept, design and testing of a fiber optic viewing system that records flame images, wavelength spectra and flicker frequencies from inside an operating gas turbine combustor. The image, wavelength spectrum and flicker frequencies give a new type of signature of the combustor that can be related to certain engine operating conditions and malfunctions. Flame data along with other engine and sensor data taken over long periods of time and under different engine conditions can give signatures to help diagnose developing, impending or suddenly occurring problems in the engine hot gas path. The diagnosis of problems should help prevent further or additional damage to the engine or predict overhaul schedules. We would also hope to identify unusual operating conditions or modes that could be avoided in normal operation. A program has been underway with the Electric Power Research Institute to develop such a viewing system. The program has been through planned stages of conceptual design, fabrication, testing in a high pressure combustor rig and is currently being field tested on a utility gas turbine at Houston Light and Power. The flame image is taken from the combustor at the distal end of a small diameter probe with a fixed wide angle field-of-view lens. Fiber optics transfers the image outside the combustor to a nearby video camera. The video image is digitized and calculations of magnitude, position, size and orientation are made on the flame image pattern. Contour plots and differences between image patterns can also be generated for further analysis. Changes in wavelength and flicker frequencies can be recorded and the flicker signal can be cross correlated with acoustic signals from the same combustor. During the rig testing phase of the program several modifications were made to the combustor to simulate problems. These modifications included: generation of air flow nonuniformities, fuel nozzle changes, simulated liner damage, fuel, water injection and air flow changes. Some results from these tests and the field engine tests will be presented.