This book focuses on the characteristics of optical radiation, or a spectrum, emitted by various plasmas. In plasma, the same atomic species can produce quite different spectra, or colours, depending on the nature of the plasma. This book gives a theoretical framework by which a particular spectrum can be interpreted correctly and coherently. The uniqueness of the book lies in its comprehensive treatment of the intensity distribution of spectral lines and the population density distribution among the atomic levels in plasmas. It is intended to provide beginners with a good perspective of the field, laying out the physics in an extremely clear manner and starting from an elementary level. A useful feature of the book is the asterisked sections and chapters which can be skipped by readers who only wish to gain a quick and basic introduction to plasma spectroscopy. It will also be useful to researchers working actively in the field, acting as a guide for carrying out experiments and interpreting experimental observations.