Understanding Lasers is a introduction to laser technology, which starts by introducing the basic principles behind lasers, how they work, and optics. It then covers important lasers, their characteristics and accessories, and concludes with an introduction to applications of lasers. In this 4th edition, the author has incorporated a new chapter, "fiber lasers and amplifiers," new topics such as the physics of optical fibers and fiber lasers, disk lasers, and Ytterbium lasers, and several new sections including "laser geometry and implications", "diode laser structures", "optimal parametric sources", and "3D printing and additive manufacturing".