Spiral Model

The spiral model, proposed by Barry Boehm in 1986, is a software development methodology that frames each project phase as a cycle of risk analysis and prototype construction. Unlike the linear waterfall model, the spiral model explicitly recognizes that the primary challenge in large software projects is not specification but uncertainty — uncertainty about requirements, technology, staffing, and market conditions. Each loop of the spiral identifies the highest-risk element, builds a prototype to resolve it, and uses the resulting information to plan the next loop.

The spiral model is iterative development with a meta-cognitive layer: it iterates not just on the product but on the project's own risk profile. It is the methodological ancestor of modern agile approaches, though its formal rigor and emphasis on documentation distinguish it from the lightweight practices that followed.