Consisting of just two atoms, diatomic molecules include some of our most abundant elements: hydrogen, nitrogen and oxygen. In this book the authors describe how quantum mechanics can be used to predict diatomic molecule spectra in a gaseous state by discussing the calculation of their spectral line intensities. Summarizing more than 30 years of quantitative analysis of temporally and spatially-resolved experimental records, and introducing insights that are essential in utilizing the inherent symmetries associated with diatomic molecules, this is a valuable reference to any academic engaged in the field of spectroscopy and serves as a comprehensive guide to anyone with a genuine interest in the subject.