The book provides an exhaustive account of all available investigations of the high frequency dynamics of liquids performed by Inelastic X-Ray Scattering (IXS). It covers a period ranging from the first development of this technique, at the end of the past millennium, to the present. A first chapter provides an historical overview of experimental probes available across the years before the actual development of IXS. The following chapter introduces the theoretical foundations of IXS, describes existing IXS spectrometers and discusses their complementarity with other probes. The next chapter provides an overview of various theoretical frameworks necessary to interpret IXS experiments, such as memory function approaches, mode-coupling theories, etc. The successive chapters review the various IXS results on liquids obtained across the years and focus on topics still open in the field. Finally the future is discussed in the last part of the book.