Some explanation of the history and authorship of this book is in order. I began writing this book while on sabbatical leave from the University of Michigan in 1975. I had chosen to spend that year at Stanford University to learn firsthand about the DENDRAL Project. My interest derived from a continuing desire to understand what has been accomplished by the field of artificial intelligence, with which I have long been associated but toward which I have tried to maintain a critical stance. DENDRAL is widely claimed to be one of the most notable successes of this field. I wondered what generalizable lessons it had to share.