This book shows software developers the powerful set of tools (CLOS) available for developing object-oriented programs in Common Lisp. The advantages of using CLOS to design, develop, maintain, and extend complex programs are many: it allows for modular implementation; programs more closely resemble what they model; programs are conveniently extensible; and CLOS defines conventions that will be shared across the Common Lisp community. CLOS has been adopted as part of Common Lisp by the X3J13 committee, which is working on creating the ANSI Standard Common Lisp. Common Lisp programmers can use this book to start developing object-oriented programs in Common Lisp as quickly as possible. Readers who are not familiar with Common Lisp can discover from this book the major themes of object-oriented programming.