The continuous scaling of transistors in the last half of century has been the driving force for electronics. The channel length of the transistors in production today is below 100nm. A wide variety of devices are also being explored to complement or even replace silicon transistors at molecular scales. Similarities between nanoscale and micronscale transistors exist, but nanotransistors also behave in drastically different ways. For example, ballistic transport and quantum effects become much more important. To push MOSFETs to their scaling limits and to explore devices that may complement or even replace them at molecular scale, a clear understanding of device physics at nanometer scale is necessary. The book provides a description of the recent development of theory, modeling, and simulation of nanotransistors for engineers and scientists working on nanoscale devices. Simple physical pictures and semi-analytical models, which were validated by detailed numerical simulations, are provided for both evolutionary and revolutionary nanotransistors.