Recent developments in high-resolution transmission electron microscopy (HRTEM) provide new possibilities for imaging—with atomic resolution—transition metal catalysts during exposure to reactive gases at elevated temperatures. The capability provides unprecedented insight into the structure, morphology, and dynamics of complex supported metal nanocluster catalysts with various surfaces and interfaces, even as they are functioning. Here we review recent work that exploits this capability of HRTEM to elucidate the nature of the exposed surfaces in such catalysts. The observations provide information about the exposed metal–surface sites, metal–support interface, and surface dynamic processes induced by changes in the gas environment. Examples include the location of a barium promoter in a ruthenium catalyst for the ammonia synthesis catalyst, gas-induced shape changes in supported copper nanocrystals for the methanol synthesis, and carbon nanofiber formation in steam reforming catalysts.