In this work, the current state of understanding of grain structure evolution during friction-stir welding is briefly reviewed. The broad aspects of this process and experimental techniques for its examination are critically addressed. The specific character of the microstructural evolutions in body-centered cubic, face-centered cubic and hexagonal close-packed metals are considered in details. In all cases, the grain structure evolution is shown to be a relatively complex process, which usually involves geometric effect of strain, continuous recrystallization and discontinuous recrystallization. Moreover, mechanical twinning, annealing twinning and grain convergence may also occur in particular cases. It is also demonstrated that activation of a specific microstructural mechanism is primarily governed by crystal structure and stacking fault energy but may also be influenced by welding temperature. Specifically, microstructure evolution in cubic metals with high stacking-fault energy is primarily governed by the continuous recrystallization whereas grain structure development in materials with low stacking-fault energy is mainly driven by the discontinuous recrystallization. In the case of transient stacking-fault energy, the materials may experience a transition from the continuous to the discontinuous mechanism. In hexagonal metals, microstructural changes are shown to be directly linked with crystallographic texture. Specifically, a formation of very sharp texture may promote the grain convergence.