Object-oriented programming (OOP) tends to improve software quality by promoting structure, extensibility, and reusability of software. Its fundamentals are data abstraction, inheritance and dynamic binding. But it is not enough to understand these concepts; one must also learn how to make good use of them. This book covers the basic concepts of OOP, schows typical application patterns, gives useful design hints, and finally presents the design and implementation of an object-oriented windows system with an integrated text and graphics editor. The language used throughout this book in Oberon-2, a clean and type-safe language designed at ETH Zรผrich. However, the emphasis of the book is not on the language but on the concepts of OOP. They can easiliy be transferred to any other object-oriented language. The book is aimed at students of computer science as well as at practitioners who want to gain a perspective on modern software development techniques. Compilers for Oberon-2 as well as the source code of the case study in this book are freely available for several common workstations. The book is aimed at students of computer science as well as at practitioners who want to gain a perspective on modern software development techniques.