Two-dimensional cristalline domains in lipid monolayers at the air/water interface are observed by fluorescence microscopy. They represent a solid at an interface between mediums with high and low dielectric constants. As the lipids exhibit noncompensated dipole moments and partly are charged, long-range electrostatic forces exist. These are varied via external electric fields leading to attraction or repulsion of the observed lipid crystals. Analysing the movement of crystallites in varying fields yields information on the molecular origin of forces and allows estimate of their magnitude. In addition the growth of crystallites within confined areas can be studied by choosing suitable field geometries.