Nanoscale catalysts are important for electrocatalysis, especially in energy conversion as in photoelectrochemical cells and fuel cells. Understanding their reactivity is essential for improving their performances and designing new ones, but challenging due to their inherent structural heterogeneity. This article reviews recent developments in using single-molecule fluorescence microscopy to overcome this challenge and interrogate directly the individuality of nanoscale catalysts. Using electrocatalysis by single-walled carbon nanotubes (SWNTs) as an example, this article discusses how the single-molecule approach dissects the reaction kinetics at single-reaction resolution, unravels the reaction mechanism, and quantifies the reactivity and inhomogeneity of individual SWNT reactive sites, which are imaged to nanometre precision via super-resolution optical imaging. New scientific questions and opportunities are also discussed, as well as the related optical studies of single-molecule and single-nanoparticle electrochemistry.