The use of X-ray inspection has evolved into an integral process to aid in the design and testing of in-pile instrumentation. Two types of X-ray inspection, three dimensional computed tomography (3D CT) and radioscopy, have been employed at the Idaho National Laboratory (INL) High Temperature Test Laboratory (HTTL). Early in the development of the high temperature irradiation resistant thermocouple (HTIR TC), radioscopy, which produces a two dimensional X-ray image or digital radiograph, was key in development of the HTIR TC. Radiographs were originally produced using an image intensifier linked to a CCD camera. Later upgrades to the radioscopy process replaced the image intensifier and CCD camera with a flat panel detector. With the increased dynamic range of the flat panel detector, additional discoveries were made about the performance of the HTIR TC. Three dimensional computed tomography is a recent tool added to the arsenal of nondestructive evaluations performed at the HTTL. This capability has enabled the development of new in-pile instrumentation to a level that would not have been achievable without this X-ray inspection process. Examples include the diamond temperature sensor, the transient hot wire thermal conductivity probe, the ultrasonic thermometer and the micro pocket fission detector. This paper will discuss the evolution X-ray inspections at the HTTL and their contribution to the development of in-pile instrumentation.