The study of drug action has benefitted greatly from the development and use of in vivo model systems. In model systems, manipulations and observations can be more rigorously controlled and screens of novel therapeutic agents can be more safely conducted. No single model system provides all of the possible advantages. At one end, mammalian models allow the study of complex behavioral patterns and the most complex of cognitive functioning. At the other end, models using simple organisms such as C. elegans allow the application of the most sophisticated and recent molecular biology and other innovative techniques. The major purpose of the present book is to highlight another model?one that we believe occupies a uniquely important position.