Metal nanoparticles are key electrode materials in a variety of electrochemical applications including basic electron-transfer study, electrochemical sensing, and electrochemical surface enhanced Raman spectroscopy (SERS). Metal nanoparticles have also been extensively applied to electrocatalytic processes in recent years due to their high catalytic activity and large surface areas. Because the catalytic activity of metal nanoparticle is often highly dependent on their size, shape, surface ligands, and so forth, methods for examining and better understanding the correlation between particle structure and function are of great utility in the development of more efficient catalytic systems. Despite considerable progress in this field, the understanding of the structure-activity relationships remains challenging in nanoparticle-based electrochemistry and electrocatalysis due to limitations associated with traditional ensemble measurements. One of the major issues is the ensemble averaging of the electrocatalytic response which occurs over a very large number of nanoparticles of various sizes and shapes. Additionally, the electrochemical response can also be greatly affected by properties of the ensemble itself, such as the particle spacing. The ability to directly measure kinetics of electrochemical reactions at structurally well-characterized single nanoparticles opens up new possibilities in many important areas including nanoscale electrochemistry, electrochemical sensing, and nanoparticle electrocatalysis. When a macroscopic electrode is placed in a solution containing redox molecules and metal nanoparticles, random collision and adsorption of nanoparticles occurs at the electrode surface in addition to redox reactions when a suitable potential is present on the electrode. In a special case where particles are catalytically more active than the substrate, the faradaic signals can be greatly amplified on particle surfaces and a steady shift in the baseline current would be expected due to many particles adsorbing on the electrode. Single particle events can be temporally resolved when an ultramicroelectrode (UME) is used as the recording electrode. The use of an UME not only reduces the collision frequency, but also greatly decreases baseline noise, thereby resulting in clear resolution of single collision events. Single particle collision has quickly grown into a popular electroanalytical technique in recent years. Alternatively, one can use nanoelectrodes to immobilize single nanoparticles so that they can be individually studied in electrochemistry and electrocatalysis. Nanoparticle immobilization also allows one to obtain detailed structural information on the same particles and offers enormous potential for developing more comprehensive understanding of the structure-function relationship in nanoparticle-based electrocatalysts. This Account summarizes recent electrochemical experiments of single metal nanoparticles which have been performed by our group using both of these schemes.