In　this　review,　we　briefly　introduce　our　in　situ　atomic-scale　mechanical　experimental　technique　(ASMET)　for　transmission　electron　microscopy　(TEM),　which　can　observe　the　atomic-scale　deformation　dynamics　of　materials.　This　in　situ　mechanical　testing　technique　allows　the　deformation　of　TEM　samples　through　a　simultaneous　double-tilt　function,　making　atomic-scale　mechanical　microscopy　feasible.　This　methodology　is　generally　applicable　to　thin　films,　nanowires　(NWs),　tubes　and　regular　TEM　samples　to　allow　investigation　of　the　dynamics　of　mechanically　stressed　samples　at　the　atomic　scale.　We　show　several　examples　of　this　technique　applied　to　Pt　and　Cu　single/polycrystalline　specimens.　The　in　situ　atomic-scale　observation　revealed　that　when　the　feature　size　of　these　materials　approaches　the　nano-scale,　they　often　exhibit　＂unusual＂　deformation　behaviours　compared　to　their　bulk　counterparts.　For　example,　in　Cu　single-crystalline　NWs,　the　elastic-plastic　transition　is　size-dependent.　An　ultra-large　elastic　strain　of　7.2%,　which　approaches　the　theoretical　elasticity　limit,　can　be　achieved　as　the　diameter　of　the　NWs　decreases　to　similar　to　6nm.　The　crossover　plasticity　transition　from　full　dislocations　to　partial　dislocations　and　twins　was　also　discovered　as　the　diameter　of　the　single-crystalline　Cu　NWs　decreased.　For　Pt　nanocrystals　(NC),　the　long-standing　uncertainties　of　atomic-scale　plastic　deformation　mechanisms　in　NC　materials　(grain　size　G　less　than　15nm)　were　clarified.　For　larger　grains　with　G＜similar　to　10nm,　we　frequently　observed　movements　and　interactions　of　cross-grain　full　dislocations.　For　G　between　6　and　10nm,　stacking　faults　resulting　from　partial　dislocations　become　more　frequent.　For　G＜similar　to　6nm,　the　plasticity　mechanism　transforms　from　a　mode　of　cross-grain　dislocation　to　a　collective　grain　rotation　mechanism.　This　grain　rotation　process　is　mediated　by　grain　boundary　(GB)　dislocations　with　the　assistance　of　GB　diffusion　and　shuffling.　These　in　situ　atomic-scale　images　provide　a　direct　demonstration　that　grain　rotation,　through　the　evolution　of　the　misorientation　angle　between　neighbouring　grains,　can　be　quantitatively　assessed　by　the　dislocation　content　within　the　grain　boundaries.　In　combination　with　the　revolutionary　Cs-corrected　sub-angstrom　imaging　technologies　developed　by　Urban　et　al.,　the　opportunities　for　experimental　mechanics　at　the　atomic　scale　are　emerging.　(C)　2014　The　Authors.　Published　by　Elsevier　B.V.