The　process　of　heterogeneous　catalytic　reaction　under　working　conditions　has　long　been　considered　a　“black　box”,　which　is　mainly　because　of　the　difficulties　in　directly　characterizing　the　structural　changes　of　catalysts　at　the　atomic　level　during　catalytic　reactions.　The　development　of　in　situ　transmission　electron　microscopy　(TEM)　techniques　offers　opportunities　for　introducing　a　realistic　chemical　reaction　environment　in　TEM,　making　it　possible　to　uncover　the　mystery　of　catalytic　reactions.　In　this　article,　we　present　a　comprehensive　overview　of　the　application　of　in　situ　TEM　techniques　in　heterogeneous　catalysis,　highlighting　its　utility　for　observing　gas-solid　and　liquid-solid　reactions　during　thermal　catalysis,　electrocatalysis,　and　photocatalysis.　in　situ　TEM　has　a　unique　advantage　in　revealing　the　complex　structural　changes　of　catalysts　during　chemical　reactions.　Revealing　the　real-time　dynamic　structure　during　reaction　processes　is　crucial　for　understanding　the　intricate　relationship　between　catalyst　structure　and　its　catalytic　performance.　Finally,　we　present　a　perspective　on　the　future　challenges　and　opportunities　of　in　situ　TEM　in　heterogeneous　catalysis.　©　2023　The　Author(s)