Tunnel inspection is a challenging problem because of high-volume traffic and routine operations in naturally aggressive environments. The need to keep tunnels open during inspection and minimize tunnel closures and user delays must be carefully balanced with the need to conduct in-depth lining inspections to ensure the safety of drivers. This paper describes the laboratory validation and field performance of a recently developed in-depth nondestructive testing technology for the detection of impairments in tunnel lining: linear array shear wave tomography, typically referred to as ultrasonic tomography (UST). Before this equipment is used in the field, the system is first evaluated through the use of laboratory specimens with artificial defects that mock common structural problems, such as air- and water-filled voids, delaminations, and other potential abnormalities. The device is also used to determine concrete thickness and reinforcement depth and spacing. The test results are discussed to determine the device's capabilities and limitations in locating defects in concrete structures. After the system is evaluated on the basis of its ability to detect these simulated defects, the system is taken to the field to inspect a public tunnel for natural structural defects. Potential regions of interest are first identified through high-speed air-coupled ground-penetrating radar (GPR) and visual inspection and subsequently inspected with UST. This paper shows that the combination of preliminary inspection procedures (GPR surveys and visual inspection) and an in-depth technique like UST is powerful for the assessment of the condition of tunnel linings and can detect potential anomalies, such as delamination, reinforcement depth and layout, and lining thickness.