The waterfall model is a breakdown of project activities into linear sequential phases, where each phase depends on the deliverables of the previous one and corresponds to a specialization of tasks. The approach is typical for certain areas of engineering design software development, it tends to be among the less iterative and flexible approaches, as progress flows in largely one direction ("downwards" like a waterfall) through the phases of conception, initiation, analysis, design, construction, testing, deployment and maintenance.


  • Time spent early in the software production cycle can reduce costs at later stages. For example, a problem found in the early stages (such as requirements specification) is cheaper to fix than the same bug found later on in the process (by a factor of 50 to 200).
  • In common practice, waterfall methodologies result in a project schedule with 20–40% of the time invested for the first two phases, 30–40% of the time to coding, and the rest dedicated to testing and implementation. The actual project organization needs to be highly structured. Most medium and large projects will include a detailed set of procedures and controls, which regulate every process on the project.
  • A further argument for the waterfall model is that it places emphasis on documentation (such as requirements documents and design documents) as well as source code. In less thoroughly designed and documented methodologies, knowledge is lost if team members leave before the project is completed, and it may be difficult for a project to recover from the loss. If a fully working design document is present (as is the intent of Big Design Up Front and the waterfall model), new team members or even entirely new teams should be able to familiarise themselves by reading the documents.
  • The waterfall model provides a structured approach; the model itself progresses linearly through discrete, easily understandable and explainable phases and thus is easy to understand; it also provides easily identifiable milestones in the development process. It is perhaps for this reason that the waterfall model is used as a beginning example of a development model in many software engineering texts and courses.
  • It is argued that the waterfall model can be suited to projects where requirements and scope are fixed, the product itself is firm and stable, and the technology is clearly understood.


  • Clients may not know exactly what their requirements are before they see working software and so change their requirements, leading to redesign, redevelopment, and retesting, and increased costs.
  • Designers may not be aware of future difficulties when designing a new software product or feature, in which case it is better to revise the design than persist in a design that does not account for any newly discovered constraints, requirements, or problems.
  • Organizations may attempt to deal with a lack of concrete requirements from clients by employing systems analysts to examine existing manual systems and analyse what they do and how they might be replaced. However, in practice, it is difficult to sustain a strict separation between systems analysis and programming.This is because implementing any non-trivial system will almost inevitably expose issues and edge cases that the systems analyst did not consider.
  • In response to the perceived problems with the pure waterfall model, modified waterfall models were introduced, such as "Sashimi (Waterfall with Overlapping Phases), Waterfall with Subprojects, and Waterfall with Risk Reduction".
  • Some organizations, such as the United States Department of Defense, now have a stated preference against waterfall-type methodologies, starting with MIL-STD-498, which encourages evolutionary acquisition and Iterative and Incremental Development.
  • While advocates of agile software development argue the waterfall model is an ineffective process for developing software, some sceptics suggest that the waterfall model is a false argument used purely to market alternative development methodologies.
  • Rational Unified Process (RUP) phases acknowledge the programmatic need for milestones, for keeping a project on track, but encourage iterations (especially within Disciplines) within the Phases. RUP Phases are often referred to as "waterfall-like"
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