Abstract: Since its invention in the 20th century, lasers have permeated every aspect of our daily life. With its exceptional spectral features it is now an indispensable light source, and lasers have been widely adopted in many areas of scientific endeavor, including laser photolithography,3D printing, laser medicine, and large-scale gravitational wave detectors based on laser interferometry.In particular, lasers with ultra-narrow linewidth and high frequency stability act as unique "eyes" that enable us to peer through the micro world of atoms and molecules. This type of laser is used to probe the spectral features of atoms or ions and can be locked onto their transitions,leading to the development of an "optical atomic clock" with unprecedented precision in timekeeping.In fact, the optical atomic clock is incapable of accumulating just one wrong second if it started to tick at the moment of the creation of the universe some 13.7 billion years ago. How to obtain such a laser is by no means a trivial task, as it involves many key scientific technologies. This paper reviews the principles and historic development of laser frequency stabilization. We also describe many important applications and future developments of ultra-stable lasers.