Ultrasonic monitoring with permanently installed sensors at high temperatures (>300°C) is not possible with conventional piezo electric transducers as they reach their Curie temperature and lose the ability to convert electrical into mechanical signals and vice versa. High temperature piezo electric materials exist but differential material temperature expansion coefficients make it difficult to permanently bond them to a substrate that is to be monitored without them peeling off when exposed to large temperature differences. To circumvent these problems the authors have developed a waveguide system that can be clamped to a component that is to be tested. The thin and slender waveguides isolate a conventional piezo-electric transducer at one of its ends from the high temperature environment at the other end. A guided wave is sent along the waveguide and coupled into the material that is to be tested; a different waveguide can then be used to pick up the return signal. Due to the waveguide geometry, temperature gradients of 100s of °C per 100mm can be maintained by natural convection cooling in air. This paper presents the waveguide system and a particular application focused on monitoring of cracks in thick steel plates at elevated temperatures.