The rapid progress in crystal growth and microfabrication technologies over the past two decades have led to the development of novel semiconductor devices. Among the most significant of these are resonant tunnelling diodes (RTDs), and this book is the first to give a comprehensive description of the physics and applications of these devices. The RTD, which utilises electron-wave resonance in double potential barriers, has emerged as one of the most important testing grounds for modern theories of transport physics, and is central to the development of new types of semiconductor nanostructure. The opening chapters of the book set out the basic principles of coherent tunnelling theory and the various fundamental concepts necessary for the study of RTDs. Longitudinal-optical phonon-assisted resonant tunnelling, the effects of impurity scattering, femtosecond dynamics, non-equilibrium distribution, space charge build-up and intrinsic bistabilities are then described in detail. The applications of RTDs, such as in high-frequency signal generation, high-speed switching, and multi-valued data storage are reviewed, and the book closes with a chapter devoted to the new field of resonant tunnelling through laterally confined zero-dimensional structures. Covering all the key theoretical and experimental aspects of this active area of research, the book will be of great value to graduate students of quantum transport physics and device engineering, as well as to researchers in both these fields.