  This brief review introduces the notion of a relativistic multifluid system
-- a multi-component system with identifiable relative flows -- and outlines a
set of models for scenarios relevant for different astronomical observation
channels. The specific problems used to illustrate the key principles include
superfluid hydrodynamics (with relevance for radio and x-ray pulsar timing and
gravitational-wave searches), heat flow (connecting to the problem of neutron
star cooling and associated x-ray observations) and the coupling between matter
and electromagnetism (linking to explosive phenomena like gamma-ray bursts and
more subtle issues like the long-term evolution of a neutron star's magnetic
field). We also comment on the coupling between matter and radiation, for which
the multifluid approach would seem less appropriate. The main motivation of the
survey is to illustrate less familiar aspects that come into play in multifluid
problems, establish the relevant "language" and provide a platform for more
detailed work on these issues.
