It is now something of a truism to say that natural toxins form a major compo­ nent of the molecular tools used increasingly frequently by the ever growing number of laboratories of various kinds. Evidence for this is provided not only by the increasing number of firms including such toxins in their catalogues but also by the large number of demands received by those who discover new tox­ ins. This book is designed to facilitate the work of scientists interested in explor­ ing a new domain of toxicology or in using toxins as tools for research. The length of this book has been kept reasonable by concentrating on animal tox­ ins, most of which are polypeptides. The venom of each animal contains a large number of toxins, which may be similar in terms of their physical and chemical properties and are therefore difficult to purify. However, their pharmacological activities may be very differ­ ent. These toxins are extremely active, and it is therefore necessary to demon­ strate conclusively that the activity observed really does correspond to the newly characterized molecule.