This book was written for the benefit of young researchers in diverse disciplines ranging from experimental plasma physics to astrophysics, and graduate students wanting to enter the interdisciplinary area of research now generally called plasma spectroscopy. The author has attempted to develop the theoretical foundations of the numerous applications of plasma spectroscopy from first principles. However, some familiarity with atomic structure and collision calculations, with quantum-mechanical perturbation theory and with statistical mechanics of plasmas is assumed. The emphasis is on the quantitative emission spectroscopy of atoms and ions immersed in high-temperature plasmas and in weak radiation fields, where multi-photon processes are not important.