  In this chapter we review recent experimental and theoretical work on various
novel superfluid phases in fermion systems, that result from pairing fermions
of different species with unequal densities. After briefly reviewing existing
experimental work in superconductors subject to a strong magnetic field and
trapped cold fermionic atom systems, we discuss how to characterize the
possible pairing phases based on their symmetry properties, and the
structure/topology of the Fermi surface(s) formed by the unpaired fermions due
to the density imbalance. We also discuss possible experimental probes that can
be used to directly detect the structure of the superfluid order parameter in
superconductors and trapped cold atom systems, which may establish the presence
of some of these phases unambiguously.
