Since its development toward the end of the past millennium, high-resolution Inelastic X-Ray Scattering (IXS) has substantially improved our knowledge of the collective dynamics of liquids at mesoscopic scales, that is, over distances and time-lapses approaching those typical of first neighboring atoms' interactions. However, despite the undoubted scientific relevance and the rapid evolution toward maturity, comprehensive monographs on this technique are not available. The primary purpose of this book is to partially fill this lack while providing a helpful reference for both mature scientists and less experienced researchers in the field.After a general introduction to the fundamental aspects of scattering measurements, the IXS cross-section is analytically derived, and the complementarity with Inelastic Neutron Scattering is discussed in detail.The remainder of the book reviews representative IXS studies on simple fluids focusing on topics as relevant as the dynamic crossover from the hydrodynamic to the kinetic regime, the onset of relaxation phenomena and related high-frequency viscoelasticity, the gradual emergence of quantum effects, the evidence of dynamic boundaries partitioning the supercritical domain, the prevalence of solid-like aspects in the high-frequency dynamics of fluids, and the dynamic fingerprints of the polymorphic nature of liquid aggregates.