Friction stir welding (FSW) and friction stir processing (FSP) are unique solid-state joining and surface modification techniques that offer several advantages over conventional fusion welding and surface treatment methods. The ability to modify the microstructure and properties of the weld and surface through the control of processing parameters is a key advantage of FSW/FSP. This has led to a great deal of research on understanding the microstructural evolution during FSW/FSP. One important aspect of microstructural control in FSW/P is the development of grain structures and textures. The high strain rates and temperatures during FSW/P can lead to dynamic recrystallization and the formation of refined and homogeneous grain structures. The degree of grain refinement and texture development can be controlled through the selection of processing parameters such as tool geometry, rotation speed, traverse speed, and applied pressure. Another important aspect of microstructural control in FSW/P is the formation of phases, phase transformations, and precipitation. The ability to control grain structures, phases, and IMCs, as well as the potential for local refinement and the formation of MMCs, makes FSW/P an attractive option for a wide range of applications. Continued research and development will be critical for further advancing the capabilities and potential of FSW/P.