Procedural languages are generally well understood and their formal foundations cast in the forms of various lambda-calculi. For object- oriented languages however the situation is not as clear-cut. In this book the authors propose and develop a different approach by developing object calculi in which objects are treated as primitives. Using object calculi,the authors are able to explain both the semantics of objects and their typing rules and demonstrate how to develop all of the most important concepts of object-oriented programming languages: self, dynamic dispatch, classes, inheritance, protected and private methods, prototyping, subtyping, covariance and contravariance, and method specialization. Many researchers and graduate students will find this an important development of the underpinnings of object-oriented programming.