A discussion of the structure of ivory (the dentin of the tusks of ancient and modern elephants and also walruses) and the various substances which have been used as ivory, or to simulate ivory, including the tusks and teeth of other mammals, hard ivory-like palm nuts, bone, antler, horn, celluloid and other polymers. The macroscopic and microscopic features that most clearly distinguish one from another are explained, primalrily in the context to aid conservators in making identifications, but also to illuminate the behavior and degradation of the materials. -- AATA