This book enables readers to achieve ultra-low energy digital system performance.  The author's main focus is the energy consumption of microcontroller architectures in digital (sub)-systems.  The book covers a broad range of topics extensively: from circuits through design strategy to system architectures. The result is a set of techniques and a context to realize minimum energy digital systems. Several prototype silicon implementations are discussed, which put the proposed techniques to the test. The achieved results demonstrate an extraordinary combination of variation-resilience, high speed performance and ultra-low energy.