A molecular device is an assemblage of a discrete number of molecular components (that is, a supramolecular structure) designed to achieve a specific function. Each molecular component performs a single act, while the entire supramolecular structure performs a more complex function, which results from the cooperation of the various molecular components. Molecular devices operate via electronic or nuclear rearrangements. Like any device, they need energy to operate and signals to communicate with the operator. The extension of the concept of a device, so common on a macromolecular level, to the molecular level is of interest not only for basic research but also for the growth of nanoscience and nanotechnology.