Object-oriented programming is a powerful paradigm for constructing reusable and maintainable code. This book gives an introduction to object-oriented programming in the R programming language. Object-oriented programming is a style of programming that focuses on data as "objects" that have state and can be manipulated by polymorphic or generic methods.In object-oriented programming, you model your programs by describing which states an object can be in and how methods will reveal or modify that state. Object-oriented programming achieves high flexibility through so-called polymorphism, where which concrete methods are executed depends on the type of data being manipulated.In this book, I teach you how to write object-oriented programs. How to construct classes and class hierarchies in the three object-oriented systems available in the R programming language, and how to exploit polymorphism to write flexible and extendable software.