My contribution to this book reflects the help and advice of many people. I am particularly indebted to the people at Prentice Hall who diligently worked to bring this book to market. Thanks to all of my students at Columbia University, who perennially demonstrated that teaching is learning. Thanks to Carol Brennan for her valuable comments and suggestions. To Ben Rosenzweig and Douglas Scherer, thanks for making this project a rewarding and enjoyable experience. Special thanks to David Dawson, whose insightful ideas and sincere support encourage me to work hard to the very end. Most important, thanks to my mom and dad, Natalia and Victor, whose excitement, enthusiasm , inspiration, and support were exceeded only by their love.