**1. Introduction**

In recent years, on the basis of FSP/FSW technologies, a number of new solutions, methods and variants have been developed, constituting not only proof of the continuous evolution of FSP/FSW technologies, but also of the huge scientific and application potential hidden in these methods. The key idea of these works was to induce more beneficial changes in the microstructure of materials and their properties, as well as to ensure better control of the processes taking place at individual stages of machining [1,2]. As is known, the main condition to obtain the assumed material characteristics is appropriate selection of the process parameters and knowledge of the changes in the microstructure of the material and its properties as a function of the machining parameters [3], shape and dimensions of the tool [4,5] or, for example, the sample cooling method [6–9]. That is why systematic research in this area and experimental verification of developed concepts and ideas are so important. The key objective of this Special Issue was to present the current state of knowledge and the recent advances in developing the microstructure and properties of materials using modern FSP and related technologies. The Special Issue made it possible to collect valuable articles presenting the results of research on changes in the microstructure and properties of the material caused by the application of the above technologies, as well as presenting the cognitive and application potential in addition to underlining the implementation problems and challenges faced by the users of FSP and related technologies.
