Most descriptions of development processes are based on the Waterfall model, where all stages of development follow each other. Requirements must be fixed at the start and at the end we get a Big Bang delivery. In practice hardly anybody really follows this model, although in reporting to management, practice is bent into this model, because management usually expects this simple model, while most development procedures describe it as mandatory. This causes a lot of mis-communication and thus wastes a lot of energy.

Requirements Architecture Design Code Test The waterfall model Delivery

Early descriptions of Evolutionary delivery, then called Incremental delivery, are described by Harlan Mills in 1971 and F.P. Brooks in his famous "No silver bullet" article in 1987 [SC2]. Incremental delivery is also used in Cleanroom Software Engineering [SP8, SP11, SP12, SP20]. A practical elaboration of Evolutionary development theory (Evo) is written by Tom Gilb in his book Principles of Software Engineering Management in 1988 [D4] and in newer manuscripts on Tom's web-site: http://www.gilb.com. Incremental delivery is also part of eXtreme Programming (XP), see http://www.extremeprogramming.org, however, if people claim to follow XP, I hardly see the Evo element practiced to its successful potential.

A recent article by Craig Larman and Victor Basili: "Iterative and Incremental Development: A Brief History" was published in 2003, showing that the application of Evo methods dates back to the mid-50s.

We prefer using the expression Evolutionary delivery, or Evo, as proposed by Tom Gilb, because not all Incremental delivery is Evolutionary. Sometimes we may find Spiral development, however, there are several spiral development descriptions, like from Boehm (1988) and in the NASA Software Management Guidebook [N4]. Incremental delivery methods use more or less cycles, where in each cycle part of the design and implementation is done. However, not always emphasis is put on typical Evolutionary issues, like: