Metal particle size is an important parameter to consider when looking at supported catalyst performances. As for other properties, surface reactivity of metallic nanoparticles is thus highly size-dependent. The smallest size that can be reached for this type of object is of course the isolated atom deposited on a support. The preparation of single-atom catalysts (SAC) is not trivial, since to avoid clustering, the mean adsorption energy of the metal on the support should be higher than its cohesive energy. The choice of a support that allows a strongly interacting environment with the metal is therefore a key step in the preparation of such catalysts. The spectrum of carbon (nano)materials is very broad, and their structure as well as their surface chemistry, although sometime complex, can be tuned and exploited for the stabilization of SAC. This Review summarizes in a comprehensive manner experimental and computational studies aiming at: i) preparing SAC on carbon materials, ii) understanding the metal-support interactions in SAC, and iii) studying how this relates to catalytic performances. The use of SAC follows well the needs dictated by society (energy and environment issues), and many studies have already been published in various fields, such as thermal catalysis, photocatalysis and electrocatalysis.