The Allan Variance is perhaps the most famous of David Allan’s achievements, with its derivative for telecom: TDEV, the Time Deviation. He 'wrote the book' on the methods for characterising clocks and time and frequency distribution systems and the statistical variance he introduced is named after him. David Allan is as well recognized in the field of time and frequency technology as any other figure in the world. He has been a major figure in the world of Time and Frequency since the 70's and 80's when he led the NBS (National Bureau of Standards, since renamed NIST-- National Institute of Standards) developments of the time scale measurement system and algorithm which are still used as among the best in the world. His achievements are historically numerous and international, and he continues to this day to be an active leader with a number of recent contributions. David has been honoured as a major contributor throughout Europe, the USA, Russia and the former Soviet Union, and in China. His current work is on an oven-less quartz oscillator that performs as an atomic clock, and his new Unified Field Theory.