  The hydrodynamic description of a superfluid is usually based on a two-fluid
picture. In this thesis, basic properties of such a relativistic two-fluid
system are derived from the underlying microscopic physics of a complex scalar
quantum field theory. To obtain analytic results of all non-dissipative
hydrodynamic quantities in terms of field theoretic variables, calculations are
first carried out in a low-temperature and weak-coupling approximation. In a
second step, the 2-particle-irreducible formalism is applied: This formalism
allows for a numerical evaluation of the hydrodynamic parameters for all
temperatures below the critical temperature. In addition, a system of two
coupled superfluids is studied. As an application, the velocities of first and
second sound in the presence of a superflow are calculated. The results show
that first (second) sound evolves from a density (temperature) wave at low
temperatures to a temperature (density) wave at high temperatures. This role
reversal is investigated for ultra-relativistic and near-nonrelativistic
systems for zero and nonzero superflow. The studies carried out in this thesis
are of a very general nature as one does not have to specify the system for
which the microscopic field theory is an effective description. As a particular
example, superfluidity in dense quark and nuclear matter in compact stars are
discussed.
