This book covers main properties of the excitation spectrum in superfluid 4He and the thermodynamics determined by the spectrum. It deals with hydrodynamics and describes that quantitative results should be insignificantly modified with processes of phonon decay taken into account.

A classic from 1965, this book covers the main aspects of the theory of quantum liquids, including the elementary excitation spectrum, hydrodynamics, and kinetic phenomena. The book requires no special training and assumes only general knowledge of the fundamentals of theoretical physics. It was developed from studies at the Institute of Physical Problems of the U.S.S.R. Academy of Sciences and can be used as a guide for professors teaching quantum liquid theory or as a text for graduate students.