Object-Oriented Programming (OOP), in computer science, type of high-level computer language that uses self-contained, modular instruction sets for defining and manipulating aspects of a computer program. These discrete, predefined instruction sets are called objects and they may be used to define variables, data structures, and procedures for executing data operations. In OOP, objects have built-in rules for communicating with one another. By using objects as stable, preexisting building blocks, programmers can pursue their main objectives and specify tasks from the top down, manipulating or combining objects to modify existing programs and to create entirely new ones.

Object-oriented programming began with Simula, a programming language developed from 1962 to 1967 by Ole-Johan Dahl and Kristen Nygaard at the Norwegian Computing Center in Oslo, Norway. Simula introduced definitive features of OOP, including objects and inheritance. In the early 1970s Alan Kay developed Smalltalk, another early OOP language, at the Palo Alto Research Center of the Xerox Corporation. Smalltalk made revolutionary use of a graphical user interface (GUI), a feature that allows the user to select commands using a mouse. GUIs became a central feature of operating systems such as Macintosh OS and Windows.

The most popular OOP language is C++, developed by Bjarne Stroustrup at Bell Laboratories in the early 1980s. In 1995 Sun Microsystems, Inc., released Java, an OOP language that can run on most types of computers regardless of platform. In some ways Java represents a simplified version of C++ but adds other features and capabilities as well, and it is particularly well suited for writing interactive applications to be used on the World Wide Web.