In the preface from the 1979 predecessor to thOR book, Hopcroft and U11man marveled at the fact that the subject of automata had exploded, compared with its state at the time they wrote their first book, in 1969. Truly, the 1979 book contained many topics not found in the earlier work and was about twice its size. If you compare this book with the 1979 book, you will find that, like the automobiles of the 1970's, this book is" larger on the outside, but smaller on the inside." That sounds like a retrograde step, but we are happy with the changes for several reasons. First, in 1979, automata and language theory was still an area of active research. A purpose of that book was to encourage mathematically inclined students to make new contributions to the field. Today, there is little direct research in automata theory (as opposed to its applications), and thus little motivation for us to ret~ n the succinct, highly mathematical tone of the 1979 book. Second, the role of automata and language theory has changed over the past two decades. In 1979, automata was largely a graduate-level subject, and we imagined our reader was an advanced graduate student, especially those using the later chapters of the book. Today, the subject is a staple of the undergraduate curriculum. As such, the content of the book must assllme less in the way of prerequisites from the student, and therefore must provide more of the background and details of arg~ lments than did the earlier book.

A third change in the environment is that Computer Science has grown to an xlmost uvimaginable degree in the past two decades. While in 1979 it was often a challenge to 61] up a curriculum with material that we felt would survive the next wave of technology, today very many subdisciplines compete for the limited amount of space in the undergraduate curriculum. Fourthly, CS has become a more vocational subject, and there is a severe pragmatism among many of its students. We continue to believe that aspects of automata theory are essential tools in a variety of new disciplines, and we believe that the theoretical, mind-expanding exercises embodied in the typical automata course retain their value, no matter how much the student prefers to learn ouly the most imrn_ ediately monetizable technology. However, to assure a continued place for the subject on the menu of topics available to the corn-purer science student, we believe it is necessary to emphasize the applications along with the mathematics. Thus, we have replaced a number of the more abstruse topics in the earlier book with examples of how the ideas are used today. While applications of automata and language theory to compilers are now so well understood that they are normally covered in a compiler course, there are a variety of more recent uses, includin E modelchecking algorithms to verify protocols and doc~ iment-description languages that are patterned on context-free grammars.