This accessible new volume examines and evaluates the principles of programming languages from both their common and language-specific elements. Each chapter is devoted to a particular programming language issue. These issues are illustrated with an example from one of the many programming languages used today. This book stems in part from courses taught at the University of Kentucky and at the University of Wisconsin-Madison on programming language design. There are many good books that deal with the subject at an undergraduate level, but there are few that are suitable for a one semester graduate-level course. This book is my attempt to fill that gap. The goal of this course, and hence of this book, is to expose first-year graduate students to a wide range of programming language paradigms and issues, so that they can understand the literature on programming languages and even conduct research in this field. It should improve the students' appreciation of the art of designing programming languages and, to a limited degree, their skill in programming. This book does not focus on any one language, or even on a few languages; it mentions, at least in passing, over seventy languages, including well-known ones (Algol, Pascal, C, C++, LISP, Ada, FORTRAN), important but less known ones (ML, SR, Modula-3, SNOBOL), significant research languages (CLU, Alphard, Linda), and little-known languages with important concepts (Io, Go .. del). Several languages are discussed in some depth, primarily to reinforce particular programming paradigms. ML and LISP demonstrate functional programming, Smalltalk and C++ demonstrate object-oriented programming, and Prolog demonstrates logic programming. Students are expected to have taken an undergraduate course in programming languages before using this book. The first chapter includes a review of much of the material on imperative programming languages that would be covered in such a course. This review makes the book self-contained, and also makes it accessible to advanced undergraduate students. Most textbooks on programming languages cover the well-trodden areas of the field. In contrast, this book tries to go beyond the standard territory, making brief forays into regions that are under current research or that have been proposed and even rejected in the past. There are many fascinating constructs that appear in very few, if any, production programming languages. Some (like power loops) should most likely not be included in a programming language. Others (like Io continuations) are so strange that it is not clear how to program with them. Some (APL arrays) show alternative ways to structure languages. These unusual ideas are important even though they do not pass the test of current usage, because they elucidate important aspects of programming language design, and they allow students to evaluate novel concepts. Certain themes flow through the entire book. One is the interplay between what can be done at compile time and what must be deferred to runtime. Actions performed at compile time make for more efficient and less error-prone execution. Decisions deferred until runtime lead to greater flexibility. Another theme is how patterns and pattern matching play a large role in many ways in programming languages. Pattern matching is immediately important for string manipulation, but it is also critical in steering logic programming, helpful for extracting data from structures in ML, and for associating caller and callee in CSP. A third theme is the quest for uniformity. It is very much like the mathematical urge to generalize. It can be seen in polymorphism, which generalizes the concept of type, and in overloading, which begins by unifying operators and functions and then unifies disparate functions under one roof. It can be seen in the homoiconic forms of LISP, in which program and data are both presented in the same uniform way. Two organizing principles suggest themselves for a book on programming languages. The first is to deal separately with such issues as syntax, types, encapsulation, parallelism, object-oriented programming, pattern matching, dataflow, and so forth. Each section would introduce examples from all relevant languages. The other potential organizing principle is to present individual languages, more or less in full, and then to derive principles from them. This book steers a middle course. I have divided it into chapters, each of which deals primarily with one of the subjects mentioned above. Most chapters include an extended example from a particular language to set the stage. This section may introduce language-specific features not directly relevant to the subject of the chapter. The chapter then introduces related features from other languages. Because this book covers both central and unusual topics, the instructor of a course using the book should pick and choose whatever topics are of personal interest. In general, the latter parts of chapters d