The three main morphological components of fiber keratins, such as wool and hair, are the cuticula, the cortex, and the cell membrane complex. The cortex is made up of cells that contain microfibrils. The microfibril matrix complex largely determines the mechanical and physical properties of the fiber keratin. A comprehensive review of x-ray, microscopic, and thermoanalysis studies of the microscopic and submicroscopic structure of the fiber keratins is presented. Elongation, the build-up of helices in the microfibril matrix complex, chemically modified fiber keratins, and heat damaged fiber keratins are considered.