What is a Risk Breakdown Structure (RBS)?
The risk breakdown structure is a hierarchical chart of risk categories — technical, external, organizational, project management — each broken into subcategories. It's the WBS idea applied to sources of risk instead of scope.
Its job is completeness during risk identification: walking the branches ("have we thought about regulatory? currency? vendor stability?") surfaces risks a free-form brainstorm misses. Don't confuse it with the resource breakdown structure, which unfortunately shares the acronym.
Worked example
In a desalination-plant risk workshop, the facilitator walks the RBS branch by branch. Under "external → regulatory," the team surfaces a pending discharge-permit change nobody had mentioned; under "technical → interfaces," the SCADA integration risk appears. The structure found what the brainstorm forgot.