This is a completely revised edition of a book originally published 25 years ago. While the general content, layout, and style of this edition follows that of the original, the second edition reflects the enormous expansion of theoretical and experimental research that has taken place over the past quarter of a century. The book, now available in paperback, is an introductory rather than a specialist work aimed at first year-graduate students of physics and is accessible to those who have taken undergraduate courses in mathematics and statistical thermodynamics. Each chapter ends with a short set of references for further reading, and exercises are provided for the reader. This volume covers the 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 of liquids fall under each topic. The author takes an atomic or molecular view of liquids, concentrating on the pair distribution and pair potential functions, illustrating how properties can be calculated from first principles and compared with experimental results. The experimental techniques used involve scattering both for equilibrium and dynamic measurements.