Writing an introductory textbook is a very challenging and intriguing task, for one is confronted with the need to find order and coherence in a vast, sprawling body of information that does not fit together easily. While this looks like my third attempt at such a synthesis, it is probably closer to the tenth or even fifteenth because each chapter has gone through multiple revisions before it became a final manuscript. This edition took five years of research and writing. If you are familiar with previous editions, you will find that every chapter has been altered substantially. My objectives and priorities, however, remain the same. This third edition simply gave me another opportunity to move closer to my ideal.