<p>Grain-boundary (GB) engineering is a fundamental tool in materials design. Ever-increasing power to resolve the atomistic structure of GBs has enabled the discovery of new GB phases and their transitions. GB phase transitions are characterized by changes in the atomistic configuration of GBs, often together with variations in their chemical composition. Such transitions can be induced by temperature, local stress state and chemical potential of the constituent elements. In this article, we highlight some exciting new frontiers in this regard. The discussion is grouped into GB faceting (noncongruent) transitions, congruent transitions of structural units, chemical segregation-induced premelting, and chemical ordering. Based on atomic-scale characterizations, excess quantities of GBs can be derived to understand their thermodynamics. Moreover, new methodologies have been developed to characterize local properties of GBs, offering direct probes on their structure–property relationships and guiding the rational design of materials with superior structural and functional properties.</p> Graphical abstract <p></p>