This Guide provides an overview on the essential types of lasers and their key properties as well as an introduction into the most important physical and technological aspects of lasers. Apart from describing the basic principles (such as stimulated emission and the properties of optical resonators), this Guide discusses the numerous important properties of laser crystals, the impact of thermal effects on laser performance, methods of wavelength tuning and pulse generation, and laser noise. Practitioners will also gain valuable insight from remarks on laser safety and obtain new ideas about how to make the laser development process more efficient.