Since Horton in 1965, many authors have sought to aggregate different variables characterizing the state of water into a single value called Water Quality Index ( WQI ). This index is intended to facilitate the operational management of water resources and their allocation for different uses. Detailed and operational description of the main WQI calculations are here reviewed. The review contains: (1) an historical analysis of the evolution of WQI calculation methods by looking both at the choice of variables, the methods of weighting and aggregating these variables into a final single value; (2) an illustration of the contradictions observed in the final result when, on the same database, the WQI is calculated by different methods; (3) the significant progress possible via fuzzy logic to define a WQI adapted to specific water use