This is one of the first models of software development, presented by B.W.Boehm. The Waterfall model is a step-by-step method of achieving tasks. Using this model, one can get on to the next phase of development activity only after completing the current phase. Also one can go back only to the immediate previous phase.

Look at the diagram:



In the diagram you can notice that each phase of the development activity is followed by the Verification and Validation activities. One phase is completed with the testing activities, then the team proceeds to the next phase. At any point of time, we can move only one step to the immediate previous phase. For example, one cannot move from the Testing phase to the Design phase.

Advantages

1. Simple and easy to use.
2. Easy to manage due to the rigidity of the model ? each phase has specific deliverables and a review process.
3. Phases are processed and completed one at a time.
4. Works well for smaller projects where requirements are very well understood.

Disadvantages

1. Adjusting scope during the life cycle can kill a project
2. No working software is produced until late during the life cycle.
3. High amounts of risk and uncertainty.
4. Poor model for complex and object-oriented projects.
5. Poor model for long and ongoing projects.
6. Poor model where requirements are at a moderate to high risk of changing.