The recent interest in molecular dynamics (MD) simulations requires for systematic and more precise descriptions of the most commonly used lipids. For example, accurately known lipid areas are crucial when assessing intermolecular interactions taking place within biomembranes. Importantly, the complex structural dynamics found in membranes involve a balance of forces, where area is the product of these forces in the lateral direction (i.e. in-plane). In addition to playing a key role in describing membrane structure and function, knowledge of lateral lipid area is central to molecular dynamics (MD) simulations. We have recently developed a model (i.e. scattering density profile, SDP) to simultaneously analyze x-ray and neutron scattering data from fully hydrated lipid bilayers [Kučerka et al., Biophys. J. 95, 2356 (2008)]. The model is based on volumetric distribution functions that are required to obey spatial conservation, and experimental volume data are incorporated into the analysis (Figure 1). Decisions regarding the specific separation of the sub-molecular components in the model are guided by an MD simulation. Most recently, using this technique we have determined the structural parameters of fluid phase bilayers of common phosphatidylcholines with fully saturated, mixed, and branched fatty acid chains, at several temperatures [Kučerka et al., Biochim. Biophys. Acta 1808, 2761 (2011)]. Bilayer parameters, such as area per lipid and overall bilayer thickness have been obtained in conjunction with intrabilayer structural parameters (e.g. hydrocarbon region thickness).