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. In Pharo there is nothing else but objects and messages. Even Booleans are true objects.Finally Pharo is fun program and fully interactive. This book is supported by the videos of the professional Pharo MOOC: (http: //mooc.pharo.org). This material guides you step by step in the exciting world of Pharo and object-oriented programming. You can also interact with Pharoers from all over the world using Discord (http: //discord.gg/Sj2rhxn). The authors have more than 20 years experience teaching advanced design and they are part of the core Pharo team.