Waterfall

Waterfall development has been used for decades to develop large, complex systems. Waterfall development programs can take years to complete, with a significant initial time and cost commitment in which requirements are analyzed and derived, and software is designed based on those requirements.

Waterfall development is the classic top-down, milestone-driven development process. It generally consists of requirements, design, implementation, verification, and maintenance phases, as shown:

Development teams progress through these phases, and each phase must be completed prior to moving to the next. It is not uncommon for a development team to spend years in a single phase, with no production code being fielded until a significant amount of time has passed.

Waterfall projects pass through a series of gates, or reviews. These ensure that stakeholders and executives are satisfied with the state of the program, before passing to the next phase. Reviews often include: 

• System Requirements Review (SRR)
• Preliminary Design Review (PDR)
• Critical Design Review (CDR)