We have seen how abstraction is vital in helping us to cope with the complexity of large systems. Effective programming also requires organizational principles that can guide us in formulating the overall design of a program. In particular, we need strategies to help us structure large systems to be modular, meaning that they divide naturally into coherent parts that can be separately developed and maintained.

One powerful technique for creating modular programs is to incorporate data that may change state over time. In this way, a single data object can represent something that evolves independently of the rest of the program. The behavior of a changing object may be influenced by its history, just like an entity in the world. Adding state to data is a central ingredient of a paradigm called object-oriented programming.