Object-Oriented Programming: Theory and Practice is a comprehensive textbook for programmers who are learning about object-oriented programming (OOP). The book presents the theory behind this interesting programming paradigm using two different widely used OOP languages, Python and Java. First, a history of OOP is presented with basic concepts. Next, the basic unit of OOP, classes and their associated constructors is covered. Then, the concept of inheritance is explored in its various forms, followed by abstraction, interfaces, and polymorphism. As a counterbalance to inheritance, the concept of composition and delegation are introduced, followed by the encapsulation of classes. Object equality, copying and persistence are covered, and lastly the subject of generics is discussed. In each chapter the subject matter is presented from a theoretical perspective followed by multiple programming examples and exercises at the end of the chapter. The 87 example programs in Python and Java are available online.