Biological membranes have unique lipid compositions suggesting a specific role for many lipids. Evidence is reviewed concerning the intermolecular forces between glycero- and sphingolipids and cholesterol, the dependence of many of these interactions on the state of ionization of lipids, pH, ionic strength, and divalent cation concentration. The effect of intermolecular interactions between certain lipids on lipid clustering, interaction with cholesterol, on the conformation of proteins, and on transitions to the hexagonal phase is considered. Other forces which cause lipids phase separation or clustering are discussed. It is concluded that lipids are in dynamic equilibrium with their environment and can act as receptors for certain intra- or extra-cellular stimuli, which they can translate into a response by undergoing changes in fluidity, phase transitions, or phase separation.