Proteins in living systems carry out a great variety of specific functions, each of which depends on the precise three-dimensional structure of a particular protein. Proteins are synthesized in the form of a flexible polypeptide chain that is capable of assuming a vast number of configurations; the transformation of this chain into a specific, relatively rigid three-dimensional structure is called folding--a remarkable process of self-organization. It is known that the amino acid sequences of some proteins have sufficient information to determine their three-dimensional structures. There are other proteins whose folding requires additional information beyond that found in the sequence of the mature protein. This book introduces the central problem of folding mechanisms as well as a number of other closely related issues. This book is neither a textbook nor a treatise. Rather, it is an attempt by several investigators to convey the excitement and challenges of those aspects of the folding problem in which they are actively engaged. The contributors give brief introductions to protein folding from the perspectives of molecular architecture, stability and dynamics, phage genetics, DNA exons, general physiology, and natural selection. They point out emerging new directions, including the suggestion of a class of diseases that result from protein folding defects.