This book focuses on two related topics in fluid mechanics, that is, speed of sound through various media and critical flow associated with gas–liquid flows. The first part of the book explores the speed of sound in liquids, gases, and solids. For gas flows, both real gas behavior and two-phase flow are reviewed; a thorough review is given of cubic equations of state. For media of solid, transverse and longitudinal waves are discussed and related back to earlier work on acoustics and gas dynamics. The second part of the book focuses on critical flow. Beginning with ideal, perfect gases, it goes into van der Waals and other equations of state. A general discussion of gas–liquid flows is subsequently given with necessary nomenclature consistent with the work of Wallis, and lastly goes on to explore critical flow for two-phase and binary mixtures through Henry and Fauskes work and the Omega method by Leung.