Electrochemistry at individual metal nanoparticles (NPs) can provide new insights into their electrocatalytic behavior. Herein, the electrochemical activity of single AuNPs attached to the catalytically inert carbon surface is mapped by using extremely small (! 3 nm radius) polished nanoelectrodes as tips in the scanning electrochemical microscope (SECM). The use of such small probes resulted in the spatial resolution significantly higher than in previously reported electrochemical images. The currents produced by either rapid electron transfer or the electrocatalytic hydrogen evolution reaction at a single 10 or 20 nm NP were measured and quantitatively analyzed. The developed methodology should be useful for studying the effects of nanoparticle size, geometry, and surface attachment on electrocatalytic activity in real-world application environment.Electrochemistry of metal nanoparticles (NPs) has been subject of numerous recent studies because of their extensive applications in electrocatalysis and sensing. [1][2][3] The catalytic activity of NPs and the reaction pathway often depend strongly on the NP shape, size, and orientation on the surface. [4][5][6] To investigate the effects of these factors, one has to visualize and measure electrochemical activity at the level of single NPs and crystal-surface facets of a particle. Optical techniques, including surface plasmon resonance imaging, [7] and single-molecule fluorescence imaging [8] were employed recently to map the catalytic activity distribution on a single NP level. One electrochemical approach to single NP experiments is to measure the current at a metal NP either landing at or attached to a small electrode. [9][10][11][12][13][14][15] The landing experiments provided more information about transport processes and collision dynamics, the size distribution and concentration of NPs than electron transfer (ET) or catalytic activities. The problems in NP immobilization experiments [13][14][15] include difficulties in characterizing the geometry of the nanoelectrode/NP system, significant background current produced by the underlying electrode surface, and poorly defined NP shape if it is formed in situ by electrodeposition.