Modern Computers are faster, more compact and more complex than their predecessors at the cost of increased power consumption. Energy from the inputs not transferred to the outputs is dissipated as heat and in irreversible circuits, while erasing a single bit of information, according to Landauer's Principle the bit energy is dissipated as heat. So, some information about the inputs is erased every time a logic operation is performed in irreversible manner. Reversible logic is a scheme where this energy dissipation due to irreversibility is minimized or sometimes almost nullified by utilizing the bit energy of previous results. In this book the concept has been illustrated and applied to construct Ultra Low Power Arithmetic Logic Circuits.