The book is divided into five parts with 16 chapters. The first part is an introductory part that includes 2 chapters, which is about the invention of lasers, overview, and applications summary. The second part has 2 chapters and is about the emission of light, atomic leap and typical lasers. The third part contains 2 chapters on resonant cavity and laser amplification. The fourth part includes 8 chapters about characteristics of laser beams, including: longitudinal mode and monochromaticity of laser, bandwidth of laser, travelling and intensity distribution of laser beam, polarization of laser, stability of continuous lasers, pulse characteristics of laser, output power of continuous-wave operation of laser, competition of laser modes, and tuning of laser cavity length. The fifth part has 2 chapters, which talk about functional lasers, laser oscillation and sensing.