Alzheimer’s disease is characterized by amorphous deposits in the brain, with the major component being the 40- to 43-residue amyloid beta (Ab) peptide. [1, 2] In its oligomeric form, Ab is believed to be a causal agent for Alzheimer’s disease. [3] While incubation studies require millimoles of Ab for aggregate formation, only micromolar concentrations are needed in the presence of lipid vesicles, [4–6] suggesting a possible role of lipids in Ab aggregation. This has led us to examine the interaction between Ab40 and three lipids with identical tails, but different head groups: zwitterionic dipalmitoyl phosphatidyl choline (DPPC), anionic dipalmitoyl phosphatidyl glycerol (DPPG), and cationic dipalmitoyl trimethyl ammonium propane (DPTAP). Although cationic lipids are absent in cell membranes, the use of DPTAP is critical to understanding the role of charge on peptide association. To mimic the outer leaflet of a membrane, monolayers at the air–water interface were held at a (bilayer equivalent [7] ) pressure of 30 mN m � 1 . [8] Our previous isotherm data show that Ab inserts into all three lipids at surface pressures below 25 mN m � 1 at 308C. [9] At 30 mN m � 1 , there is insertion for DPPG and DPTAP, but not for DPPC. Fluorescence microscopy (FM) measurements show that Ab insertion disrupts condensed domains, increasing the area fraction of the disordered phase. However, FM and isotherm measurements do not provide molecular-scale details. Here, we report the first X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD) on lipid/Ab systems. Not only do these data pinpoint the location of Ab in the lipid matrix, they also unveil an intriguing templating effect of DPPG on Ab. We first examined Ab at the bare air–water interface. From