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.