Over the past twenty years, the science and engineering of nanomechanical systems (NEMS) has developed into a quite extended and rapidly growing area of research. The first examples of such mechanical systems, advanced by the application of semiconductor lithographic techniques to the fabrication of mechanically active devices, included cantilevers where the application of a force as small as a few piconewtons would cause measurable displacements, and structures fabricated with size scales such that the fundamental mechanical mode was in the 100 MHz band. Extensive efforts exploring different materials and methods to support these structures has supported the advent of very high quality factor mechanical resonators with frequencies ranging from the MHz band well into the GHz band of frequencies. This has allowed the development of nanomechanical resonators as time-keeping systems competitive with macroscale quartz crystals; as radiofrequency filters for the cellphone industry; and increasingly as systems with strong potential for fundamental experiments in quantum mechanics as well as applications to quantum information technology. Nanomechanical systems also are playing an increasingly important and central role as ultrasensitive detectors of mass, displacement, acceleration, force or spin. The applications that have become possible include measurements of forces between individual biomolecules, forces originating in the magnetic resonant response of single electron and nuclear spins, and noise that arises from mass fluctuations involving single molecules. As a result, this area of research attracts a large number of researchers from around the world. By their nature, nanomechanical systems are interdisciplinary, since they can couple to electrical circuits or optical cavities and they have potential applications in sensing, telecommunications, biophysics, and photonics, topics which are studied not only in condensed matter but also in the applied physics.