Silk is a protein-based, usually fibrous, material produced by many invertebrates. It can be used to catch or subdue prey, protect the animal and/or its eggs, or for defence. Each type of silk has its own unique set of properties, which makes certain silks useful for human uses. One type of silk in particular, that produced by the mulberry silkworm moth, has been used for millennia as a fibre for developing luxurious textiles and apparel. Silk and the animals that produce it are thus very curious. This book overviews the diversity of silk-producing animals, comparing the types of silks produced by each of them and their functions, properties, and secretory mechanisms. The properties of each type of silk are explained by examining the chemistry of the proteins. Having established the mechanism of silk performance, the book investigates the applications of different silks, both throughout history and into the future, with explanations on how silk production is proceeding in the age of genetic engineering. Of particular mention is spider dragline (or major ampullate) silk, as it the silk considered the toughest of the silks, and is of research interest to the author.