some key elements that we associate with cell membranes. In addition, despite the crude models, the importance of hydrogen bonding and lipid-lipid interactions was recognized early on, and infrared spectroscopy, among other techniques, played an important role in supporting a dynamic fluid-like bilayer view that we still associate with cell membranes today . The more influential "fluid-mosaic model" was developed in the 1970's by Singer and Nicolson (Figure , middle) . Within this picture, membrane proteins are arranged in oriented amphipathic structures partially embedded within the phospholipid matrix, which is organized as a continuous "fluid" bilayer. This was based on X-ray crystallography, freeze-fracture electron microscopy, bulk thermodynamic measurements, and other biochemical studies. While this model has obvious limitations, the basic concept of transmembrane proteins embedded within the bulk bilayer is sufficient to explain many observations, and thus, the fluid-mosaic model remains currently relevant as a qualitative description of the local membrane environment . Initial studies using infrared spectroscopy played, perhaps the most important role, in establishing that the lipids are characterized by liquid-like environments, despite having solid-like properties at the macroscopic scale . These measurements informed the "fluid" character of the supporting lipids in the fluid-mosaic model, suggesting a dynamic bilayer environment.