This is a completely revised and rewritten edition of a book originally published 25 years ago. While the general content, layout, and style of this edition follow that of the original, the book now reflects the enormous expansion of theoretical and experimental research that has taken place over the past quarter of a century. The book is an introductory rather than a specialist work. It aims to cover the simpler and more basic parts of this vast topic, while preserving a balance between different areas. The author breaks the subject down by liquid state topic rather than by class of liquid, so that all classes can fall under each topic. The author takes a simple atomic view of liquids, concentrating on the pair distribution and pair potential functions, illustrating how properties can be calculated from first principles and the results compared with experiment. The experimental techniques used involve scattering both for equilibrium and dynamic measurements, and considerable attention is devoted to this aspect. The book is aimed at first year graduate students of physics and those who have taken undergraduate courses in mathematics and statistical thermodynamics. This edition also includes sets of exercises for the student.