This book is an introduction to the CLOS model of object-oriented programming. CLOS, the Common Lisp Object System, is a newly designed object-oriented programming language that has evolved as a standard from various object-oriented extensions of the basic Lisp language. The language definition of CLOS comprises a set of tools for developing object-oriented programs in Common Lisp. The book serves two purposes: it is a practical guide to CLOS programming and stands as a tutorial teaching object-oriented techniques for software design and development.