Nanoscale devices differ from larger microscale devices because they depend on the physical phenomena and effects that are central to their operation. This textbook illuminates the behavior of nanoscale devices by connecting them to the electronic, as well as magnetic, optical and mechanical properties, which fundamentally affect nanoscale devices in fascinating ways. Their small size means that an understanding of the phenomena measured is even more important, as their effects are so dominant and the changes in scale of underlying energetics and response are significant. Examples of these include classical effects such as single electron effects, quantum effects such as the states accessible as well as their properties; ensemble effects ranging from consequences of the laws of numbers to changes in properties arising from different magnitudes of the interactions, and others. These interactions, with the limits on size, make their physical behavior interesting, important and useful.