Two groups of information are mandatory in the domain of water pollution control: ( 1) characterization of the conditions of surface and underground waters as ecosystems or human resources on the grounds of continuous observations of pertinent chemical and physical parameters; (2) measurements for assessing the load of natural waters with pollutants from tributaries and sewerage systems. Variability of most parameters in function of time (smallest in aquifers, largest in sewers or small polluted rivers) has to be emphasized in the layout of monitoring and sampling systems. Some examples of presently used devices and the needs for new developments are discussed. Programming of intermittent information gathering as well as evaluation and condensation of series of observations have to be based on rigorous statistical considerations. Much more information is needed in this respect.