In this report I review the origins of the unit membrane theory in detail, including certain relevant descriptions of peripheral nerve structure. Discussion of the chemical composition of membranes particularly emphasizes nerve myelin and frog retinal rod outer segments. Additional discussion focuses on recent biophysical studies of myelin and of retinal rod outer segments leading to accurate high resolution electron density plots, the general problem of lipid and protein interactions in model systems and in membranes, and finally, special macromolecular arrays in certain membranes. I have considered the general problem of membranes models from the standpoint of current knowledge of the chemical composition of membranes, and have described future directions of research in the field. Treatment of the subject is mainly based on information derived from electron microscopy and x-ray diffraction techniques.