  The present review analyzes and compares various types of dissipationless
spin transport: (1) Superfluid transport, when the spin-current state is a
metastable state (a local but not the absolute minimum in the parameter space).
(2) Ballistic spin transport, when spin is transported without losses simply
because sources of dissipation are very weak. (3) Equilibrium spin currents,
i.e., genuine persistent currents. (4) Spin currents in the spin Hall effect.
Since superfluidity is frequently connected with Bose condensation, recent
debates about magnon Bose condensation are also reviewed. For any type of spin
currents simplest models were chosen for discussion in order to concentrate on
concepts rather than details of numerous models. The various hurdles on the way
of using the concept of spin current (absence of the spin-conservation law,
ambiguity of spin current definition, etc.) were analyzed. The final conclusion
is that the spin-current concept can be developed in a fully consistent manner,
and is a useful language for description of various phenomena in spin dynamics.
