Major classes of fibrous protein materials found in nature include 1) fibroin (silk) which has a structure of the simpler amino acids - glycine, alanine, and the hydroxylated amino acids - serine, tyrosine and 2) the keratins (wool, hair, feathers, horn, etc.) which are composed of much greater amounts of cystine and the more complex amino acids. Details of the structure of hair, wool, horn, feather, and silk fibers are briefly discussed.