Theory of Quantum Fluids is a concise report on the microscopic description of liquid 4He and liquid 3He in the physical density range using simple forms of the potential function between pairs of neutral atoms and the properties of the ground states and limited ranges of low excited states.The monograph covers the properties of the radial distribution function and the three-particle distribution particle; the classical sound field and the correspondence principle; paired phonon states in the free- phonon approximation; the uniform limit and the charged boson system; and the microscopic theory of a single 3He atom in the 4He liquid.Theoretical and experimental physicists will find the book very interesting.