Chemistry　of　a　catalyst　surface　during　catalysis　is　crucial　for　a　fundamental　understanding　of　mechanism　of　a　catalytic　reaction　performed　on　the　catalyst　in　the　gas　or　liquid　phase.　Due　to　the　pressure-or　molecular　density-dependent　entropy　contribution　of　gas　or　liquid　phase　of　the　reactants　and　the　potential　formation　of　a　catalyst　surface　during　catalysis　different　from　that　observed　in　an　ex　situ　condition,　the　characterization　of　the　surface　of　a　catalyst　under　reaction　conditions　and　during　catalysis　can　be　significant　and　even　necessary　for　understanding　the　catalytic　mechanism　at　a　molecular　level.　Electron-based　analytical　techniques　are　challenging　for　studying　catalyst　nanoparticles　in　the　gas　or　liquid　phase　although　they　are　necessary　techniques　to　employ.　Instrumentation　and　further　development　of　these　electron-based　techniques　have　now　made　in　situ/operando　studies　of　catalysts　possible.　New　insights　into　the　chemistry　and　structure　of　catalyst　nanoparticles　have　been　uncovered　over　the　last　decades.　Herein,　the　origin　of　the　differences　between　ex　situ　and　in　situ/operando　studies　of　catalysts,　and　the　technical　challenges　faced　as　well　as　the　corresponding　instrumentation　and　innovations　utilized　for　characterizing　catalysts　under　reaction　conditions　and　during　catalysis,　are　discussed.　The　restructuring　of　catalyst　surfaces　driven　by　the　pressure　of　reactant(s)　around　a　catalyst,　restructuring　in　reactant(s)　driven　by　reaction　temperature　and　restructuring　during　catalysis　are　also　reviewed　herein.　The　remaining　challenges　and　possible　solutions　are　briefly　discussed.