This fascinating book 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 and the effect this has on science. The effort to master time culminated in the development of modern atomic clocks. The most accurate clocks ever constructed. These advanced devices are accurate to one second in 15 billion years and now act as the primary standard for time and frequency measurement. Our improvements in the accurate measurement of time 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, the precise measurement of time and frequency afforded by atomic clocks, and other mechanisms, is being used to examine key questions about the fundamentals of our universe, such as the possibility of symmetry violation or the idea that there may be variation of the fundamental constants over time.