The paper gives an overview on interactions between two opposing interfaces in liquids. The interfaces are either solid (two silica surfaces) or fluid (in foam films or in membrane stacks). The liquids are either simple ones (water or salty water) or contain polyelectrolytes. In the latter case, the confinement leads to a structuring of the polyelectrolytes which leads to oscillatory forces in addition to DLVO forces. The effect of interface properties on the interactions is studied by modification of interfaces. In case of solid interfaces, they are modified by the adsorption of polyelectrolyte multilayers, while fluid interfaces are varied by changing the amphiphile/polymer combination. With regard to the studied problem, different methods are used to study the interactions.