Guided wave testing  Guided wave testing (GWT) is a non-destructive evaluation method. The method employs acoustic waves that propagate along an elongated structure while guided by its boundaries. This allows the waves to travel a long distance with little loss in energy. Nowadays, GWT is widely used to inspect and screen many engineering structures, particularly for the inspection of metallic pipelines around the world. In some cases, hundreds of meters can be inspected from a single location. There are also some applications for inspecting rail tracks, rods and metal plate structures. Although guided wave testing is also commonly known as guided wave ultrasonic testing (GWUT) or ultrasonic guided waves (UGWs) or long range ultrasonic testing (LRUT), it is fundamentally very different from conventional ultrasonic testing. The frequency used in the inspection depends on the thickness of the structure, but guided wave testing typically uses ultrasonic frequencies in the range of 10 kHz to several MHz. Higher frequencies can be used in some cases, but detection range is significantly reduced. In addition, the underlying physics of guided waves is more complex than bulk waves. Much of the theoretical background has been addressed in a separate article. In this article, the practical aspect of GWT will be discussed. The study of guided waves propagating in a structure can be traced back to as early as the 1920s, mainly inspired by the field of seismology. Since then, there has been an increased effort on the analytical study of guided wave propagation in cylindrical structures. It was only in the early 1990s that guided wave testing was considered as a practical method for the  non-destructive testing of engineering structures. Today, GWT is applied as an integrated health monitoring program in the oil, gas and chemical industries. Unlike conventional ultrasonics, there are an infinite number of guided wave modes that exist for a pipe geometry, and they can be generally grouped into three families, namely the torsional, longitudinal and flexural modes. The acoustic properties of these wave modes are a function of the pipe geometry, the material and the frequency. Predicting these properties of the wave modes often relies on heavy mathematical modeling which is typically presented in graphical plots called dispersion curves. In the guided wave testing of pipelines, an array of low frequency transducers is attached around the circumference of the pipe to generate an axially symmetric wave that propagates along the pipe in both the forward and backward directions of the transducer array. The torsional wave mode is most commonly used, although there is limited use of the longitudinal mode.