Ultrasonic techniques are widely used in industry for thickness gauging, flaw detection and materials characterization. The ultrasonic waves are usually generated and detected by piezoelectric transducers and coupled to the inspected part either by direct contact or through a water bath or a water jet. Although widespread and generally cost effective, these conventional ultrasonic techniques suffer from essentially two severe limitations, which impact upon their use for on-line process control and the inspection of advanced materials.