Grain　boundary　(GB)　plastic　deformation　is　a　critical　deformation　mode　that　can　significantly　affect　the　microstructure　and　mechanical　properties　of　metals.　Given　its　importance,　numerous　studies　have　been　conducted　over　the　past　few　decades　to　investigate　the　atomistic　mechanism　of　GB　plasticity,　which　includes　GB　migration,　GB　sliding,　and　grain　rotation.　These　three　deformation　modes　typically　occur　simultaneously　and　interact　cooperatively.　Despite　substantial　advancements　in　understanding　the　atomic-scale　mechanisms　of　GB　plasticity,　there　is　a　scarcity　of　review　papers　addressing　the　in　situ　atomic-scale　mechanisms　of　GB　plasticity.　This　work　aims　to　provide　a　comprehensive　overview　of　the　atomistic　mechanisms　of　GB　plasticity　in　metals,　detailing　modes　predicted　by　theoretical　models　and　molecular　dynamics　(MD)　simulations,　as　well　as　discussing　mechanisms　verified　by　in　situ　atomic-scale　experiments.　Additionally,　it　examines　the　factors　influencing　GB　deformation.　The　authors　intend　for　this　review　to　serve　not　only　as　a　valuable　resource　for　researchers　in　the　field　of　nanocrystalline　(NC)　metals　but　also　as　a　textbook　for　educating　graduate　students.