Phosphatidylethanolamine and Lysophosphatidylethanolamine belong to a group of phospholipids generally termed cephalins. The phospholipids are universally distributed among living organisms and are thought to be essential components of cell membranes. From the quantities of lipoidal and protein material found in cell membranes (Danielli and Stein, 1956) together with surface tension and related studies (Danielli and Harvey, 1935; Danielli and Davson, 1935), the membranes are considered to consist of a bimolecular leaflet of lipids with a unimolecular layer of protein adsorbed onto the polar surface. The course of investigation of the cephalin fraction has not been reviewed in recent years, and here is discussed the general scope of the work in this field, under the headings of preparation, structure, synthesis and physical properties.