The necessity to investigate the quality of structures and materials after the manufacturing process or as the system is aging, has increased the need for Non-Destructive Evaluation (NDE) techniques. NDE made it possible to scan the targeted object for highlighting and quantifying the presence of defects or sub-surface flaws that could jeopardize its stability. The research performed in this field aimed to evaluate the effectiveness of several type of techniques in detecting defects on a number of large-sized structures. In particular, investigations were performed using infrared thermography (IRT), Terahertz Inverse Synthetic Aperture Radar (ISAR), X-ray imaging, and photogrammetry as described in the following links: