This text is intended as an introduction to object-orientation for computer science and software engineering students or those actively involved in the software industry. It adopts a "hands on" approach providing a thorough introduction to O-O as well as to the language itself. Issues such as SmallTalk style, typical SmallTalk bugs and the way to develop a SmallTalk program are comprehensively covered, as is the testing of O-O systems. A number of chapters are devoted to design, including chapters on the Unified Modelling Language (UML), Object Modeling Technique (using the UML) and the recent Patterns work. Finally, there is a look at the future of object-orientation and SmallTalk.