The final end-product required determines the processing given to an animal skin. What can be produced is governed by the skin structure, which differs according to the animal. In leather, the three-dimensional collagen fibre network is stabilised by the tannin molecules, different tannins yelding leathers with different properties. The technology of leather making is described in depth. Collagen chemistry, the helical structure, and shrinkage temperature are described. Amino-acid contents of collagen, elastin, keratin and albumin are tabulated. Various stages of leather working, the process of tanning, and tanning agents are described. Chrome tanning is considered in detail, as well as aluminium and zirconium alternatives. Vegetable, aldehyde and oil tannage are also discussed. Some leather properties and possible mechanisms of deterioration conclude the article. Diagrams, references.