Fluid surfactant bilayers are composed of two mono-molecular sheets that are weakly held together by normal stresses. Except for viscous resistance, the monolayers are free to slide relative to one another — giving rise to “hidden” degrees of freedom within the composite structure. This thesis examines such an added level of complexity to the study of bi layer membranes. A continuum theory for monolayer-coupling is first developed, which forms the theoretical basis for the work reported here and for future analyses on stratified fluid membranes. The phenomenological coefficient of dynamic (viscous) coupling between monolayers is then measured experimentally by a novel method called nanotether extrusion. This technique, which is developed by myself for this present investigation, enables measurements of inter-monolayer viscous stresses in unsupported bilayers. Further, a useful spin-off from such a technique is the determination of bilayer bending rigidities. Finally, as an application of the present continuum model, the static and dynamic features of a bilayer vesicle’s Brownian shape undulations are predicted with the membrane con ceptualized as a stratified composite structure. Throughout this thesis, it is shown that the effects of bilayer stratification (monolayer-coupling) are very important. For example, the conformational dynamics of bilayer membranes on mesoscopic length scales (roughly between 1 to 100 nanometers) are governed by the inter-monolayer viscous forces;​ in such situations, dissipation due the surrounding hydrodynamics plays only a secondary role.