In this fascinating book Masatoshi Kajita recounts the importance of precise measurements and their inherent uncertainty, before telling the story of humankind's efforts to define and measure time with increasing accuracy, culminating in the development of atomic clocks. These improvements in the accurate measurement of time and frequency have played a pivotal role in the development of modern science; including the confirmation of Einstein's Theory of Relativity, and the recent detection of gravity waves. Furthermore, such measurements afforded by atomic clocks and other mechanisms are being used to examine key questions about the very fundamentals of our universe, the possibility of symmetry violation and even testing the idea that there may be variation of the fundamental constants themselves over time.