This book teaches you the key aspects of object-oriented programming: objects, classes, messages, methods and inheritance. This book is different from most books: - it focuses on Test Driven Design from day one. - it uses an extremely well-designed, simple, pure and powerfull object-oriented language, Pharo (http: //www.pharo.org). Why using Pharo? Because Pharo' syntax fits on a postcard and with Pharo you program a world of interacting objects...