on organizing the data to reflect the real structure of the world being modeled. Marvin Minsky and Seymour Papert formed many of our aitudes about programming and its place in our intellectual lives. To them we owe the understanding that computation provides a means of expres- sion for exploring ideas that would otherwise be too complex to deal with precisely. ey emphasize that a student’s ability to write and modify programs provides a powerful medium in which exploring be- comes a natural activity. We also strongly agree with Alan Perlis that programming is lots of fun and we had beer be careful to support the joy of programming. Part of this joy derives from observing great masters at work. We are fortu- nate to have been apprentice programmers at the feet of Bill Gosper and Richard Greenbla. It is difficult to identify all the people who have contributed to the development of our curriculum. We thank all the lecturers, recitation instructors, and tutors who have worked with us over the past fieen years and put in many extra hours on our subject, especially Bill Siebert, Albert Meyer, Joe Stoy, Randy Davis, Louis Braida, Eric Grimson, Rod Brooks, Lynn Stein and Peter Szolovits. We would like to specially ac- knowledge the outstanding teaching contributions of Franklyn Turbak, now at Wellesley his work in undergraduate instruction set a standard that we can all aspire to. We are grateful to Jerry Saltzer and Jim Miller for helping us grapple with the mysteries of concurrency, and to Peter Szolovits and David McAllester for their contributions to the exposition of nondeterministic evaluation in Chapter 4. Many people have put in significant effort presenting this material at other universities. Some of the people we have worked closely with are Jacob Katzenelson at the Technion, Hardy Mayer at the University xxvi