Decision makers must keep in mind that water services have an invaluable ‘price’ for the public budget and society’s welfare. However, due to the specific features of the service, water utilities are prone to inefficiency. Thus, performance evaluation becomes indispensable to face this constraint. Besides the application of the traditional technique of Data Envelopment Analysis to evaluate the performance of Portuguese water utilities, this study also implements the robust technique of order-m to assess the influence of operational environment on water utilities’ performance. In spite of several reforms, the Portuguese water sector still reveals significant levels of inefficiency, partially caused by an inappropriate operating scale. Concerning the operational environment, it was possible to observe a positive influence of private sector participation, economies of scope, and groundwater sources on efficiency. The influence of the ‘quality of service provided’ was also studied in this performance analysis.