This book explores the concepts of object-oriented programming, which have become the cornerstone of most programming languages. The book introduces the meaning of classes and objects, inheritance, encapsulation, and polymorphism. It also contains examples of Unified Modeling Language (UML) that enable the reader to model systems. The book explains these concepts in a simple manner and includes the application of these concepts through a large number of examples in three different programming languages: C#, VB.Net, and Python. The concepts introduced in the book are applicable to any programming language which supports object-oriented programming. The book is an indispensable resource that will enhance its readers' system development skills.