Our aim in writing this book has been to provide a unified, yet elementary, account of the theory of quantum liquids. Strictly speaking, a quantum liquid is a spatially homogeneous system of strongly interacting particles at temperatures sufficiently low that the effects of quantum statistics are important. In this category fall liquid 3He and 4He. In practice, the term is used more broadly, to include those aspects of the behavior of conduction electrons in metals and degenerate semiconductors which are not sensitive to the periodic nature of the ionic potential. The conduction electrons in a metal may thus be regarded as a normal Fermi liquid, or a superfluid Fermi liquid, depending on whether the metal in question is normal or superconducting.