As early as the 1950s, IBM programmers were working on software for things like submarine control systems and missile tracking systems, which were so complex that they could not be conceived and built in one go. Programmers had to evolve them over time, like cities, starting with a simple working system that could be tested by users, and then gradually adding more function and detail in iterative cycles that took one to six months to complete. In a 1969 IBM internal report called simply “The Programming Process,” IBM computer scientist M.M. Lehman described the approach: “The design process is… seeded by a formal definition of the system, which provides a first, executable, functional model. It is tested and further expanded through a sequence of models, that develop an increasing amount of function and an increasing amount of detail as to how that function is to be executed. Ultimately, the model becomes the system.” This iterative approach to software development, where programmers start by creating a simple, working seed system and expand it in subsequent cycles of user testing and development, has become a common approach in software design, known under a variety of names such as iterative development, successive approximation, integration engineering, the spiral model and many others, but in 2001, when a group of prominent developers codified the core principles in a document they called the Agile Manifesto, they gave it the name “agile” which seems to have stuck. Agile is about small teams that deliver real, working software at all times, get meaningful feedback from users as early as possible, and improve the product over time in iterative development cycles. Developing software in an agile way allows developers to rapidly respond to changing requirements. Agile developers believe that where uncertainty is high there is no such thing as a perfect plan, and the further ahead you plan, the more likely you are to be wrong.