Owing to their structural dispersion, the catalytic properties of nanoparticles are challenging to characterize in ensemble-averaged measurements. The single-molecule approach enables studying the catalysis of nanoparticles at the single-particle level with real-time single-turnover resolution. This article reviews our single-molecule fl uorescence studies of single Au-nanoparticle catalysis, focusing on the theoretical formulations for extracting quantitative reaction kinetics from the single-turnover resolution catalysis trajectories. We discuss the single-molecule kinetic formulism of the Langmuir-Hinshelwood mechanism for heterogeneous catalysis, as well as of the two-pathway model for product dissociation reactions. This formulism enables the quantitative evaluation of the heterogeneous reactivity and the differential selectivity of individual nanoparticles that are usually hidden in ensemble measurements. Extension of this formulism to single-molecule catalytic kinetics of oligomeric enzymes is also discussed.