Physical Foundations of Spectroscopy provides a clear and accessible introduction to the basic physical and mathematical principles upon which spectroscopy is based and explores the links between spectroscopy and subjects such as classical mechanics, thermodynamics, statistical mechanics, electricity and magnetism, relativity, and quantum mechanics. The text highlights how specific physical processes connect to spectroscopy, revisiting general principles and including discussions of many-body dynamics, conservation laws, symmetries, and transport processes. Through its exploration of spectroscopy, the book delves into the connections between the field and subjects such as particle and radiation transport, and the dynamics of electrons in gases and in tenuous and dense plasmas as they interact with internal quantum states in atomic ions. A final chapter dissects the solar spectrum as a touchstone across spectroscopy of remote objects, to cement concepts and illustrate some of the behaviour of spectral lines in terms of elementary ideas. Aimed at students and researchers in such diverse fields as astronomy, atmospheric science, plasma physics, and cosmology, the text is intended as a self-contained guide, including simplified examples and problem sets to enhance and support learning and providing a strong grounding in the field of spectroscopy.