  We first review the recent accumulating evidences of the existence of a
scalar-isoscalar meson with the mass 500 to 800 MeV which may be identified
with the sigma meson as the quantum fluctuation of the amplitude of the chiral
order parameter. We indicate that phase shift analyses which respect chiral
symmetry (ChS), analyticity and crossing symmetry of the scattering amplitude
show the sigma meson pole in the $s$-channel as well as the $\rho$ meson pole
in the $t$-channel in the pi-pi scattering in the $I=J=0$ channel. We emphasize
that the existence of the $\sigma$ resonance does not contradict with the
success of the chiral perturbation theory; phenomenological difficulties with
the renormalizable linear sigma model do not necessarily deny the validity of
the linear representation of ChS of QCD as given by the NJL-like models which
not only admit the $\sigma$ resonance but also reproduce the coupling constants
$L_i$ and $H_i$ appearing the nonlinear chiral lagrangian. We give some
examples of the hadronic phenomena which are naturally accounted for with the
$\sigma$ meson. We show that the the $\sigma$ meson as the amplitude
fluctuation of the chiral order parameter may be more clearly identified than
in free space in hot and/or dense matter, even in finite nuclei where partial
restoration of ChS may be realized.
