The structure, chemistry and physical properties of the cell membrane complex (CMC) of keratin fibers are reviewed, highlighting differences in the three types of CMC. Starting with Rogers' initial description of the CMC in animal hairs, several important developments have occurred that will be described, adding new details to this important structure in mammalian hair fibers. These developments show that essentially all of the covalently bound fatty acids of the beta layers are in the cuticle and exist as monolayers. The beta layers of the cortex are bilayers that are not covalently bonded but are attached by ionic and polar linkages on one side to the cortical cell membranes and on the other side to the delta layer. The delta layer between cortical cells consists of five sublayers; its proteins are clearly different from the delta layer that exists between cuticle cells. The cell membranes of cuticle cells are also markedly different from the cell membranes of cortical cells. Models with supporting evidence are presented for the three different types of cell membrane complex: cuticle-cuticle CMC, cuticle-cortex CMC, and cortex-cortex CMC.