| Sewers are aging, expensive assets that attract public attention only when they fail. Sewer operators are under increasing pressure to minimize their maintenance costs while preventing sewer failures. Inspection can give early warning of failures and allow economical repair under noncrisis conditions. Current inspection techniques are subjective and detect only gross defects reliably. They cannot provide the data needed to confidently plan long-term maintenance. This paper describes PIRAT, a quantitative technique for sewer inspection.