  The universal power law tails of single particle and multi-particle time
correlation functions are derived from a unifying point of view, solely using
the hydrodynamic modes of the system. The theory applies to general correlation
functions, and to systems more general than classical fluids. Moreover it is
argued that the collisional transfer part of the stress-stress correlation
function in dense classical fluids has the same long time tail $\sim
t^{-1-d/2}$ as the velocity autocorrelation function in Lorentz gases.
