This volume is intended for students and professionals in diverse areas of the biological and biochemical sciences. It is oriented to those who are unfamiliar with the use of physical methods in studies of the biological elements. We hope the reader will find the material a helpful reference for other volumes of this series as well as the general literature, and some may see ways to adopt these techniques in their own pursuits. Every effort has been made to avoid an abstruse presentation. It should be clear that one individual cannot be expert in all the disciplines considered here (and the authors recognize that fact with sin­ cere humility). As may be expected of an introductory reference, most of our attention was focused on the commonly used methods. To balance this, we have included a few examples of approaches which are promising but relatively undeveloped at this time. Also, an emphasis has been placed on element selectivity. It is impossible to envision the course of future events, and a volume which deals with instrumentation is especially prone to become outdated. Nevertheless, any valid approach to a scientific question should be applicable indefinitely.