  We present the theory of orbital ordering in orbital-degenerate itinerant
electron systems. After proposing the criterion of instability for orbital
ordering or orbital density wave ordering, we find that the orbital and the
spin-orbital collective excitation spectra in ferro-orbital ordered phase
exhibit finite gaps. The anomalous electronic energy spectra manifested in the
angle-resolved photoemission spectroscopy (ARPES) and the orbital occupation in
the resonant X-ray scattering (RXS) intensities are also presented for the
orbital-ordered phase.
