Metal-based heterogeneous catalysts mostly consist of supported nanoparticles of a few nanometers in size, since these objects provide a good compromise between exposed active surface area and structural stability. However, far from being static, heterogeneous catalysts constantly evolve under reaction conditions, thereby creating and removing surface sites in response to their gaseous or liquid reactive environment. Modern ex situ and in situ investigations have shown that nanocatalysts at work can face a number of deep restructuring phenomena, such as morphological change, compound formation, segregation, leaching, as well as redispersion and other metalsupport interaction effects. Recently emerging single-atom catalysts are-like subnanometric clusters-especially sensitive to their chemical environment. Using examples from the recent literature with a particular emphasis on operando characterization studies, this article reviews the main dynamic effects induced by the reaction medium on nanocatalysts (including nanoalloy catalysts) and single-atom catalysts.