Abstract Nanoclusters involve finite metal atoms are of great importance in chemical synthesis and catalysis due to their unique structure-dependent properties. Knowing how nanoclusters work in reaction conditions is no doubt one of the most important challenges, which requires deep insight into atomic-scale dynamics during the process. State-of-the-art in-situ transmission electron microscope (TEM) can realize the real-time study, which is expected to reveal insights into the underlying mechanisms of catalysis and design efficient catalysts. In this minireview, we aim to give a critical review and recent progress on nanocluster dynamics unveiled by in-situ and environmental TEM from two aspects: evolutions of metal single atoms and nanoclusters in high-temperature reaction conditions and the well-defined molecular clusters in confined space. We emphasize the cluster-environment interaction and the cluster structure–property relationships. In the end, we also summarize the present achievements and propose future challenges. We expect this review will fuel the excitement and stimulate research in the fields of cluster and catalysis.