  The dynamic sructure factor as the basic quantity describing the collective
excitations in a fluid is considered. We consider the cases of neutral and
Coulombic fluids. The classical method of moments is applied to construct the
dynamic structure factor satisfying all known sum rules. An interpolational
formula is found which expresses the dynamic characteristics of a classical or
quantum fluid system in terms of its static correlation parameters. The
analytical results based on the theory of moments are compared with Molecular
dynamics data for various model systems.
