This book provides a comprehensive introduction to the physics of time and time measurement, from an historical perspective to the modern day. It discusses the stability and accuracy of atomic frequency standards, including different types of oscillators and atomic clocks, covering recent developments and uses of these devices. The precision of atomic clocks and the atomic time scale are considered in the context of fundamental physical research, with relation to general relativity and applications such as the Global Positioning System. The authors also discuss International Atomic Time and its relationship to Coordinated Universal Time and the time scales used in astronomy. The book will be an ideal introduction for graduate students or researchers entering the fields of time and frequency metrology and precise astronomical observation. It will also be useful as a reference for scientists working in these and related areas, such as geophysics, atomic physics, astronomy and telecommunications.