The effects of high pressure on the phase state and molecular structure of pure lipid bilayers are discussed. The relations of <latex>$\Delta$</latex>H, <latex>$\Delta$</latex>S and <latex>$\Delta$</latex>V in phase transitions are straightforward and are discernible in heterogeneous bilayers in natural membranes. The effects of pressure on the dynamic properties of bilayer constituents are less clearly understood, but order parameters obtained at pressure by different techniques show agreement. The extent and significance of hydration is poorly understood. Four physiological functions are discussed: passive permeability, active transport, membrane excitability and synaptic transmission. It is shown that a full interpretation of the kinetic effects of pressure on these processes requires much more detailed molecular information than is available at present.