" .. .1 always worked with programming languages because it seemed to me that until you could understand those, you really couldn't understand computers. Understanding them doesn't really mean only being able to use them. A lot of people can use them without understanding them." Christopher Strachey The development of programming languages is one of the finest intellectual achievements of the new discipline called Computer Science. And yet, there is no other subject that I know of, that has such emotionalism and mystique associated with it. Thus, my attempt to write about this highly charged subject is taken with a good deal of in my role as professor I have felt the need for a caution. Nevertheless, modern treatment of this subject. Traditional books on programming languages are like abbreviated language manuals, but this book takes a fundamentally different point of view. I believe that the best possible way to study and understand today's programming languages is by focusing on a few essential concepts. These concepts form the outline for this book and include such topics as variables, expressions, statements, typing, scope, procedures, data types, exception handling and concurrency. By understanding what these concepts are and how they are realized in different programming languages, one arrives at a level of comprehension far greater than one gets by writing some programs in a xii Preface few languages. Moreover, knowledge of these concepts provides a framework for understanding future language designs.