<p>In this chapter I begin with a historical perspective of membrane models, starting in the early twentieth century. As these membrane models evolved, so did experiments to characterize the structure and water content of purified lipid bilayers. The wide-spread use of the X-ray gravimetric, or Luzzati method, is critically discussed. The main motivation of the gravimetric technique is to determine the number of water molecules/lipid, n(W), and then derive other important structural quantities, such as area/lipid, A(L). Subsequent experiments from the Nagle/Tristram-Nagle laboratory using X-ray and neutron scattering, first determine A(L) and then calculate n(W), using molecular lipid V(L) and water V(W) volumes. This chapter describes the details of our volume experiments to carefully measure V(L). Our results also determine n(W)', the steric water associated with the lipid headgroup, and how our calculated value compares to many literature values of tightly-associated headgroup water.</p
