This paper presents some issues concerning water supply systems assessment through performance indicators, based upon research work recently carried out in Portugal about water utilities. A proposal of 50 performance indicators divided into five different groups is presented here, namely structural indicators, operational indicators, water and service quality indicators, personnel indicators and economic indicators. These indicators were associated with a hierarchical structure of knowledge or development according to different levels: basic level, development level and strategic level. The performance indicators, structure, their implementation and assessment methodology and examples of some indicators, use concerning the characterisation or assessment of a water utility are also presented in the paper.