This document discusses measuring the level of corruption in the water sector. It presents Klitgaard's formula that corruption equals monopoly power plus discretion minus accountability. While corruption perception indexes provide a general picture, they are not sector-specific. The document analyzes correlation between corruption and various development indicators. It also reviews estimates of corruption's financial impact, from 15-45% increased costs. However, reliable sector research is limited. The document concludes that proxy measures comparing pre- and post-anti-corruption intervention costs may be more reliable, and recommends further estimating leakage to inform policymaking.