Nanoparticle assemblies hold great promise as new materials for catalysis, nanoelectronics, and nanophotonics applications. However, many of their properties, which depend on the relative arrangement of the particles within the assembly, are not sufficiently well-understood because of a lack of methods to systematically assemble them into well-defined discrete model systems. We here report a method which uses a minimal set of dynamic DNA templates to generate a large number of discrete gold nanoparticle assemblies. These assemblies are addressable in real time and can undergo structural switching and write/erase functions in response to external agents. More specifically, control of geometry is demonstrated by the facile creation of triangle and square gold nanoparticle assemblies; modularity is shown by positioning two different sizes of gold nanoparticles into all the possible triangular combinations; structural switching is established by the use of the same square template to selectively construct square, trapezoidal, and rectangular assemblies; and a write/erase function is shown by assembling a triangle of three gold nanoparticles, selectively removing one of the particles, followed by the "writing" of a different particle. The study of these systems promises to shed light on the phenomena of single electron transport and optical coupling in nanoparticle assemblies and will lead to the more effective incorporation of nanoparticles in photonic/electronic devices. In principle, our dynamic templates can be used to organize any DNA-labeled nanocomponent into well-defined and addressable structures, and as such, this constitutes a new and economical method to construct discrete nanoparticle materials on the nanoscale.