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V-model A software life-cycle model that is a development of the waterfall model. The diagram shows the STARTS V-model in which successive phases are displayed in a V formation with square-corner boxes representing the activities performed in a phase and with rounded-corner boxes representing the outputs from a phase. Outputs become the inputs to the next phase. The left leg of the V includes phases in which the detail of the design and the implementation of the software are gradually increased. In ascending the right leg of the V, the software is progressively assembled from its modules and testing proceeds from single modules through to the full system and eventual acceptance of the system.

Across-phase activities such as project and quality management are included; some links to contractual procedures are also included, for example invitation to tender (ITT). The model shares the weakness of waterfall-type models in omitting a diagrammatic representation of iteration between life-cycle phases. See also spiral model.