Catalytic nanomaterials play a major role in chemical conversions and energy transformations. Understanding how materials control and regulate surface reactions is a major objective for fundamental research on heterogeneous catalysts. In situ environmental transmission electron microscopy (ETEM) is a powerful technique for revealing the atomic structures of materials at elevated temperatures in the presence of reactive gases. This approach can allow the structure–reactivity relations underlying catalyst functionality to be investigated. Thus far, ETEM has been limited by the absence of in situ measurements of gas-phase catalytic products. To overcome this deficiency, operando TEM techniques are being developed that combine atomic characterization with the simultaneous measurement of catalytic products. This article provides a short review of the current status and major developments in the application of ETEM to gas-phase catalysis over the past 10 years.