Abstract: Guided waves can travel a long distance with lower attenuation. Therefore, they are applied to the rapid and effective large-scale inspection of defects and damages in the thin plates. Compared with classical ultrasound technique, nonlinear guided wave mainly focuses on the high-order harmonics generated from the nonlinear interaction of monochromatic ultrasonic wave and microstructural evolution. High-order harmonics show high sensitivity to the defects or damages, whose size are much smaller than the corresponding wavelength of fundamental waves. The second harmonic of guided wave has been widely utilized to quantitatively evaluate the early damages since they can be easily excited. However, this method cannot determine the position of the localized damage. Meanwhile, the second harmonic of ultrasonic guided waves is susceptible to the nonlinearity of the measurement system, making it difficult to distinguish the sources of nonlinearity. Corresponding shortcomings can be solved by the nonlinear mixing of guided waves since their frequencies, modes, and even the propagation directions can be flexibly selected. Up to now, theoretical, numerical and experimental investigations of the nonlinear guided wave mixing have been conducted, and they have already been utilized to assess the localized fatigue, thermal aging, micro-cracks, impact damage, and the deformation during creep. In addition, the following problems will be considered in the future, such as nonlinear mixing of guided waves at high frequencies, and the propagation properties of second-order harmonics generated from the non-collinear mixing of Lamb waves, as well as the interaction between the difference harmonics or sum harmonics and other types of damages.