Understanding the catalytic properties of nanoparticles is fundamentally important, but hampered by the intrinsic heterogeneity following from their structural dispersions and dynamics. This obstacle can be overcome if one can follow the catalysis of individual nanoparticles in real time. This article summarizes recent developments in using single-molecule fluorescence microscopy to study single nanoparticle catalysis. These studies reveal and quantify heterogeneous and dynamic behavior of individual nanoparticles that highlight the intricate interplay between catalysis, heterogeneous reactivity, variable surface sites, and surface restructuring dynamics in nanocatalysis. Challenges and future directions in single-nanoparticle catalysis research are also discussed. (C) 2009 Elsevier B. V. All rights reserved.