  We study in this paper the general properties of a many body system of
fermions in arbitrary dimensions assuming that the {\em momentum} of individual
fermions are good quantum numbers of the system. We call these systems
$k$-Fermi liquids. We show how Fermi liquid, Luttinger liquid (or Fermi liquid
with exclusion statistics) and spin-charge separation arises from this
framework. Two exactly solvable $k$-Fermi liquid models with spin-charge
separation are discussed as examples.
