Lipid compositional variation across the cell's membranes as well as asymmetry between two contiguous monolayers in the same membrane are critical for alive and healthy cells. The underlying mechanisms that govern this lipid variation, however, are unknown. The use of chemically altered lipids as well as the use of external chemical pre-treatments of membranes have led to, for example, reported half-lives for lipid flipping from several hours to several days. We have employed time resolved Small angle neutron scattering (TR-SANS) as a non-invasive tool to measure lipid transport without any such requirements. Our SANS measurements, however, can only study high curvature lipid vesicles (50-200nm). In this proposal we intend to compare flipping rates in planar lipid membranes using time resolved neutron reflectivity to those obtained by TR-SANS on the same system.