<p>Grain growth, governed by the mobility and migration mechanisms of grain boundaries, is a fundamental process shaping the microstructure and properties of polycrystalline materials. This article explores the critical interplay between solute adsorption, disconnections, and complexion transitions in determining grain-boundary behavior. Traditional models based on curvature-driven motion are insufficient, as experimental and computational studies reveal complex mechanisms, including disconnection-mediated migration and transitions in structure or interface excess concentration. Solute atoms not only exert classical drag effects, but can also catalyze or suppress mobility by altering activation energies for disconnection formation or modifying boundary structures. The concept of grain-boundary complexions provides a thermodynamic framework for understanding structural and chemical transitions at interfaces, with direct implications for mobility. Special attention is given to ionic systems, where charge effects and electric fields further influence migration. Collectively, these insights highlight new strategies for tailoring grain growth to engineer advanced materials.</p> Graphical abstract <p></p>