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 colors 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 plasma. It is intended to provide beginners with a good perspective of the field, laying out the physics in an extremely clear manner, starting from an elementary level. A very 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 very useful to researchers working actively in the field, acting as a guide for carrying out experiments and interpreting experimental observations.