Power ultrasound is used to stimulate materials with amplitudes in the range of 1–100 µm e.g., for ultrasonic welding, which uses high-frequency vibrations to create a solid-state bond between two materials. Ultrasonic welding can be used for both plastics and metals, and is a versatile and efficient method, particularly for joining dissimilar materials. Its application can be found in various fields such as aerospace, automotive, and electronic industries where weight reduction and durability are critical. The process is capable to create robust joints of high strength lightweight materials such as Ti6Al4V with CF-PEEK, highly efficient electric connections between Aluminium and Copper for battery packs or low-loss joints of hard magnetic Nd2Fe14B with 316L stainless steel. During ultrasonic metal welding, the joining partners undergo plastic deformation. This plastic deformation cannot be explained solely by the welding force or the temperature development due to the relative movement of the sonotrode and the joining partners, but rather as a reaction to the high-frequency oscillation, induced by the sonotrode and transferred through the materials. This enhanced plastic deformation is often referred to as the acoustic softening effect, which occurs when the metal is subjected to sound waves, e.g. during ultrasonic welding, and is a key mechanism for the modification of the microstructure of metals, as presented in this contribution. Microstructural modifications, phase transformations, the formation of twinned regions and grain growth have been observed in various metals. Analyses of the energy input by the process and the energy required for microstructural changes, allowed to estimate the impact of ultrasound on the materials behaviour during ultrasonic stimulation. These findings support the holistic understanding of ultrasonic processes, allowing frequencies, amplitudes, and forces to be precisely adapted to the specific requirements of the materials that need to be joined or modified by power ultrasonics.