Book excerpt: Introduction to Molecular Beams Gas Dynamics is devoted to the theory and phenomenology of supersonic molecular beams. The book describes the main physical idea and mathematical methods of the gas dynamics of molecular beams, while the detailed derivation of results and equations is accompanied by an explanation of their physical meaning. Many of the applications of supersonic molecular beams are discussed, including their application to molecular spectroscopy, and the study of surface phonons by monoatomic and monokinetic beams, and the study of intermolecular potentials and the onset of condensation. The phenomenology of supersonic beams can appear complex to those not experienced in supersonic gas dynamics and, as a result, the few existing reviews on the topic generally assume a limited level of knowledge. The book begins with a quantitative description of the fundamental laws of gas dynamics and goes on to explain such phenomena. It analyzes the evolution of the gas jet from the continuum to the regime of almost free collisions between molecules, and includes numerous figures, illustrations, tables and references.