The spiral model is a software process introduced by Barry Boehm in his 1988 IEEE Computer paper “A Spiral Model of Software Development and Enhancement.” Its defining feature, in Boehm’s words, is that “it creates a risk-driven approach to the software process rather than a primarily document-driven or code-driven process.”
Instead of running once through a fixed sequence, a project spirals outward through repeated rounds. Each round moves through the same four activities: determine objectives, alternatives, and constraints; evaluate alternatives and identify and resolve risks; develop and verify the next-level product; and plan the next phases. The radial distance from the center represents accumulated cost, and each loop ends with a review and commitment to proceed.
Boehm positioned the spiral as a synthesis that “incorporates many of the strengths of other models and resolves many of their difficulties.” It draws on the waterfall and evolutionary models but puts risk analysis at the center, so the project can use prototyping, simulation, or further analysis whenever uncertainty is high, and fall back toward more sequential steps when risks are low.
The model became influential precisely because it reframed process choice as a risk-management decision rather than a fixed methodology. It is often described as a bridge between the rigid waterfall approach and the lighter iterative and incremental methods that followed, and Boehm’s later work on the win-win spiral and anchor-point milestones extended the idea further.