  We identify the factors responsible for the appearance of energy-density
inhomogeneities in a self-gravitating fluid, and describe the evolution of
those factors from an initially homogeneous distribution. It is shown that a
specific combination of the Weyl tensor and/or local anisotropy of pressure
and/or dissipative fluxes entails the formation of energy-density
inhomogeneities. Different cases are analyzed in detail and in the particular
case of dissipative fluids, the role of relaxational processes as well as
non-local effects are brought out.
