This article reviews some recent experimental studies on film instabilities. Since surface instabilities of planar films are driven by destabilizing interfacial pressures, the patterns that form in thin films are a signature of these interfacial forces. The direct correspondence of pattern selection and the underlying destabilizing pressures provides a way to quantitatively determine these interface potentials. This principle is documented for three different film destabilization mechanisms: van der Waals forces, forces caused by an applied electric field, and forces that arise from the confinement of thermal molecular motion in the film. (c) 2005 Wiley Periodicals, Inc.