Spiral model

The Spiral model is a risk-driven software development process model introduced by Barry Boehm in 1986. It treats software development as an iterative process in which each cycle explicitly addresses the highest-risk elements of the project first. The process spirals outward through repeated cycles of risk analysis, prototyping, and stakeholder evaluation, with each cycle producing a more complete and refined system.

The spiral model was a direct response to the limitations of both the waterfall model, which assumes stable requirements, and early agile approaches, which sometimes treated risk as a secondary concern. By making risk the primary driver of process structure, the spiral model provides a disciplined framework for managing uncertainty in large-scale, safety-critical systems — defense, aerospace, medical devices — where the cost of late-discovered failure is catastrophic.

Each cycle of the spiral involves four activities: determining objectives, identifying and resolving risks, developing and verifying the next-level product, and planning the next cycle. The model is not a prescriptive methodology but a meta-model that can accommodate different development techniques within each cycle, from formal specification to rapid prototyping.