A thorough understanding of device physics at the nanoscale size is required in order to push MOSFETs to their scaling limitations and investigate devices that might supplement or even replace them at the molecular scale. For electrical engineers, physicists, and chemists working on nanoscale devices, Nanoscale Transistors presents an overview of the current developments in theory, modelling, and simulation of nanotransistors. For both traditional and novel nanotransistors, straightforward physical diagrams and semi-analytical models that were verified by thorough numerical simulations are given. After reviewing fundamental ideas, the article provides a foundation for evaluating novel devices by summarising the key components of conventional semiconductor devices, digital circuits, and systems.