This study reviews the extent to which the five Paris Principles (PPs) on Aid Effectiveness (AE) as set out in the Paris Declaration (PD) on AE: ownership, alignment, harmonisation, managing for results and mutual accountability, are being applied in the water and sanitation sector. The approach included in-depth case study research in three countries Bangladesh, Ethiopia and Uganda) and a broader document review. From this evidence the study identified ways in which external support to the water sector can be delivered more in the spirit of the PD.