  A unified description of elementary and collective excitations in quantum
Hall systems is presented within the single-mode approximation (SMA) framework,
with emphasis on revealing an intimate link with Chern-Simons theories. It is
shown that for a wide class of quantum Hall systems the SMA in general yields,
as an effective theory, a variant of the bosonic Chern-Simons theory. For
single-layer systems the effective theory agrees with the standard Chern-Simons
theory at long wavelengths whereas substantial deviations arise for collective
excitations in bilayer systems. It is suggested, in particular, that Hall-drag
experiments would be a good place to detect out-of-phase collective excitations
inherent to bilayer systems. It is also shown that the intra-Landau-level modes
bear a similarity in structure (though not in scale) to the inter-Landau-level
modes, and its implications on the composite-fermion and composite-boson
theories are discussed.
