Cell membranes are lamellar structures formed by the self-assembling of lipid bilayers associated with proteins. Due to the amphipathic nature of these molecules, they form a monomolecular layer at the air/water interface in a half membrane-like structure. These films (monolayers) can be studied in a Langmuir trough as a function of temperature, molecular area and surface pressure, rendering valuable thermodynamic data, analogous to the conventional temperature, volume and pressure phase diagrams. Langmuir troughs can be coupled to other equipment, like fluorescence and Brewster angle microscopes, as well as electrodes to measure and apply electrostatic fields. This allows observing and manipulating the membrane interface from top at the micrometer level, and studying membrane domains. Additionally, thickness at the nanometer level can be estimated. For accurate determination of the out-of-plane structure (scattering length density profile) as well the in-plane (acyl chain order), powerful x-ray scattering sources (high brilliance synchrotron beamlines) are needed. Monolayers are especially useful because they allow for manipulation and observations very difficult to achieve in other systems. On the other hand, bilayers and multilayers can be studied in aqueous suspension by using weaker x-ray (or neutron) beam sources. Although less manipulable, these are more similar to the structure of the biological membrane. In this presentation, we show results on natural systems like spinal cord myelin, mainly concerning the membrane homogeneity-heterogeneity problem, and the influence of ions and other environmental variables. The addition of Na⁺ or Ca²⁺ in myelin leads to a permanent phase separation along the compression isotherm. On the contrary, the suppression of salts leads to phase homogenization at high surface pressure. A similar trend is observed in multilayers.