The purpose fo this book is to present a comprehensive account of the physical concepts and theoretical approaches developed for the study of the dynamical properties of liquids (or more generally, of high-density fluids) at a microscopic level. After a discussion of the basic dynamical phenomena to be interupted, as well as of the various experimental probes, the book gradually exposes the reader to the sophisticated theoretical techniques needed for a satisfactory account of both single particle and coleective motions. The complications are faced in a stepwise fashion, with special attention to the physical content of the results. As a result of the progress achieved in the last decade, in the end a satisfactory understanding of most of the phenomena characterizing this fascinating field emerges.