Three major findings are reported here. First, a new technique based on lithographic modification with a scanning tunneling microscope (STM) has been developed for fabricating single Ag particles on atomically flat Au(111) surfaces. Second, surface-enhanced Raman scattering (SERS) from one isolated Ag particle has been observed and quantified for the first time. The enhancement factor due to one Ag particle of about 1 mu m diameter is on the order of 10(4). This factor includes contributions from all forms of chemical enhancement, which are smaller than 10(2). Finally strong evidence suggests the importance of clusters containing closely-spaced particles. Extra enhancement due to clusters of randomly distributed Ag particles is around 10(2). We believe that the extra enhancement is spatially localized near the gap sites between two particles in proximity. These gap sites form an important class of SERS-active sites.