This volume gives an up-to-date, systematic account of the microscopic theory of Bose-condensed fluids developed since the late 1950s. The core of the present book is the development of the field-theoretic analysis needed to deal with a Bose-condensed fluid. However, the author also brings out the essential physics behind the formal Green's function techniques and presents plausible scenarios to understand recent high resolution neutron and Raman scattering data. The first three chapters give a useful overview and introductory material while the concluding chapter gives an assessment and suggestions for further research.