Book excerpt: International Series of Monographs in Natural Philosophy, Volume 3, Interstellar Gas Dynamics focuses on the progress on research on the motion of gases in interstellar space and stellar atmospheres, including issues on cosmical gas dynamics and magnetohydrodynamics. The publication first offers information on the distribution, motion, and physical state of the interstellar gas. Topics include composition of the interstellar medium; cloud structure of the interstellar gas; spiral structure and distribution of the interstellar gas; ionization of the interstellar hydrogen; temperature of the interstellar gas; interaction between the interstellar gas and magnetic fields; and relativistic particles in an interstellar space. The book also takes a look at the discontinuities in the motion of the interstellar gas, as well as the fundamental theory of shock waves, ionization fronts, shock waves with light emission, and hydromagnetic discontinuities. The manuscript examines the equations of motion of the interstellar gas, including hydromagnetic equations of motion, one-dimensional motions and similarity solutions, motion of ionization fronts, and interstellar turbulence. The publication is a dependable reference for readers interested in interstellar gas dynamics.