Atomic force microscopy was used to investigate dewetting instabilities of liquid films from solid and liquid substrates in three limiting cases. We examined dewetting of a poly(styrene-co-acrylonitrile) (SAN) copolymer of viscosity eta(T)(SAN) from a solid substrate, SiOx, and from a liquid polystyrene (PS) substrate whose viscosity eta(s)(PS) much greater than eta(T) (SAN)I The third case involved the dewetting of PS liquid films from liquid SAN substrates of varying thickness, Hs. In this latter case, the viscosity of the top PS liquid layer eta(T)(PS) much greater than eta(s)(SAN), the viscosity of the SAN substrate layer. SAN films dewet SiOx substrates by a spinodal process when they were of thickness H < 300 Angstrom. For H > 300 Angstrom the SAN films were stable. For the situations in which the substrates were liquid PS films, SAN films of thickness H-T(SAN) < 1000 Angstrom dewet via a spinodal mechanism whereas SAN films of H-T(SAN) > 1000 Angstrom dewet the PS substrates by a mechanism involving the nucleation of holes with outer rims and the subsequent growth of these holes. Two regimes of growth were identified: an early stage regime where the hole initially nucleates and a rim is yet to form and a later stage regime where the rim exists. In this later stage, the hole radii R-SAN similar to t and the width of the rims l(SAN) similar to t(1/2). The height of the rims, q(SAN), increased at a more rapid rate with time than the width of the rims and l(SAN) much greater than q(SAN). In the final situation examined, where the PS films dewet the liquid SAN substrates (eta(T)(PS) much greater than eta(S)(SAN)) of varying thickness, we identified a transition from low-viscosity liquid substrate-like behavior to solid substrate-like behavior when the thickness of the SAN substrate layer became sufficiently thin. Finally, we discuss significant differences in the mechanisms by which holes grow when the substrate viscosity is liquidlike and when it is solidlike.