In principle, an ideal small-angle scattering experiment with respect to distribution, concentration, and structural integrity of the peptide species can yield multiple structural parameters (Fig. 1) . The reflection geometry for aligned lipid films (Fig. 1i ) is suited to determine the vertical density profile q(z) of the bilayers, averaged in the xy plane, e.g., by least-square fitting of the specular reflectivity curve (Constantin et al. 2003) . The specular peaks are accompanied by diffuse scattering in the form of so-called Bragg sheets extending along q k . This scattering contribution can be assigned to lateral inhomogeneities at mesoscopic length scales (nanometers to a few micrometers) due to membrane fluctuations-always present in the fluid/ hydrated state-and peptide insertions causing local orientational disorders (Fig. 1ii) , which break the lateral translation invariance.