Subnanometer clusters have attracted a widespread attention in the last decades since they show unique catalytic properties compared with larger NPs due to finite-size effects. In this regime the catalytic properties become size dependent to the extreme that the addition or subtraction of a single atom can have a drastic impact on them; this is the regime where each atom counts. More importantly, they possess a huge surface to volume ratio which allows a maximal utilization of atoms, i.e., the so-called atomic efficiency. It is important to note that, treating clusters as static structures has been shown to be a simplified picture. In reality, clusters display high fluxionality at usual operating temperatures and form an ensemble of statistically relevant isomers, each of which exhibits different catalytic properties and constantly interconverting in response to external stimuli. All in all, clusters present a very strong structure-activity relationship by which the catalytic properties can be tuned by physical and chemical modifications (e.g., size, composition, support, ligand capping, etc.); in short, clusters offer a myriad of possibilities for nanocatalyst design.