This chapter reviews some of the results that have been obtained in our laboratories with regard to characterization of lateral order in model bilayer membranes as well as biological membranes, with particular focus on membrane domains in the submicron regime. The results are based mainly on fluorescence microscopy and two‐photon laser scanning microscopy as well as atomic force microscopy (AFM). The chapter then shows by a specific example how a small enzyme, secretory phospholipase A₂ (s‐PLA₂) becomes activated at membrane interfaces in a way that is controlled by the lateral structure of the lipid‐bilayer substrate. This serves as a clear‐cut example of how certain collective properties of a lipid assembly are marshalling the function of a protein, hence highlighting the importance of lipids for membrane function. It furthermore shows how novel instrumentation and imaging techniques open up a new window to allow for quantitative description of biochemical processes.