Object-oriented programming is a popular buzzword these days. What is the reason for this popularity? Is object-oriented programming the solution to the software crisis or is it just a fad? Is it a simple evolutionary step or a radical change in software methodology? What is the central idea behind object-orien ted design? Are there special applications for which object-oriented program ming is particularly suited? Which object-oriented language should be used? There is no simple answer to these questions. Although object-oriented programming was invented more than twenty years ago, we still cannot claim that we know everything about this programming technique. Many new con cepts have been developed during the past decade, and new applications and implications of object-oriented programming are constantly being discovered. This book can only try to explain the nature of object-oriented program ming in as much detail as possible. It should serve three purposes. First, it is intended as an introduction to the basic concepts of object-oriented program ming. Second, the book describes the concept of prototypes and explains why and how they can improve the way in which object-oriented programs are developed. Third, it introduces the programming language Omega, an object oriented language that was designed with easy, safe and efficient software development in mind.